ADU Warning Signs: A San Francisco Homeowner's Reference Guide

ADU Warning Signs: A San Francisco Homeowner’s Reference Guide

A hairline crack in new stucco costs under $400 to seal. Left through one rainy season in San Francisco’s wet, fog-heavy climate, that same crack can invite moisture intrusion that costs $8,000 to remediate, and here’s the part nobody tells you: the visible crack often disappears while the damage accelerates. This guide ranks every ADU warning sign by how fast it compounds and what it costs to fix at 30 days versus 6 months. We want you to make real decisions, not just worry. By the end, you’ll know which signs demand a call tonight, which ones can wait until spring, and which ones are simply normal settling in a San Francisco ADU.

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Quick Answer

The ADU warning signs that demand immediate attention in San Francisco are: water staining on ceilings or walls, doors or windows that suddenly jam shut, any crack wider than 1/8 inch, visible mold or persistent musty odors, and electrical flicker that repeats. Most other signs, including hairline drywall cracks and minor stucco checking, are normal settling and can be monitored quarterly. A homeowner who documents signs with photos and dates, then reports them in writing to the builder, protects warranty claims under the 365-Day Done Right Promise that Sutton ADU San Francisco writes into every project.

Table of Contents

• Severity Tiers: Fix Within 48 Hours, 30 Days, or Monitor Quarterly
• Foundation and Structural Signs: What’s Settling and What’s Failing
• Moisture and Water Intrusion: The Invisible Damage Multiplier
• Garage Conversions vs. Detached ADUs vs. Attached Additions: They Fail Differently
• Which Signs Void a Builder’s Warranty If Ignored (And How to Document Reporting)
• Electrical and Plumbing Warning Signs Specific to ADUs
• Prefab and Modular ADU-Specific Signs: Cover, Dvele, Mighty Buildings, and Boxabl
• How San Francisco’s Climate Specifically Attacks ADUs
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions
• The Bottom Line

Severity Tiers: Fix Within 48 Hours, 30 Days, or Monitor Quarterly

Not every warning sign is an emergency. Treating them all as emergencies means you spend money you didn’t need to spend. Treating them all as “keep an eye on it” means a $400 fix becomes a $12,000 remediation. Here’s how we rank severity after nearly two decades building and inspecting ADUs across San Francisco, from the Richmond District to Bayview.

Tier 1: Fix Within 48 Hours

These signs indicate active water intrusion, structural movement, or electrical hazard. They compound hourly, not monthly. Each day of delay adds measurable cost.

• Water staining on any ceiling or wall surface. A stain means water has already penetrated the building envelope. The leak is active or recently active. In San Francisco’s fog belt, interior surfaces stay damp longer, which accelerates mold growth from days to hours.
• Electrical flicker that repeats in one circuit. Not a single flicker from a power surge. A circuit that flickers when you run the microwave or when the wind blows indicates a loose connection, and loose connections arc. Arcing causes fires. This needs an electrician within 48 hours.
• Any door or window that suddenly won’t open or close. Sudden is the key word. Gradual sticking over months can be seasonal wood swell. Sudden jamming means the frame or the rough opening has shifted, and that means structural movement, not humidity.
• Visible mold on any surface. Do not wait to see if it spreads. By the time mold is visible, the moisture source has been active for weeks. In San Francisco’s average 71 percent relative humidity, visible mold doubles its colony size in under 48 hours during wet months.
• The smell of gas or a persistent sewage odor. We did not include gas lines in this guide’s DIY scope for good reason. If you smell gas, call PG&E immediately, then call a licensed plumber. A sewage odor in an ADU often means a dry trap or a venting issue, but it can also mean a cracked waste line under the slab.

Tier 2: Fix Within 30 Days

These signs are not emergencies, but they are warnings. Waiting longer than 30 days lets a minor issue become a structural one, especially during San Francisco’s rainy season from November through March.

• Cracks wider than 1/8 inch in drywall, stucco, or tile. A hairline crack under 1/8 inch is almost always cosmetic settling and can be monitored. A crack wider than 1/8 inch means movement beyond what normal lumber shrinkage produces. Have it looked at within 30 days.
• Doors or windows that stick seasonally. If the sticking comes and goes with rain or heat, it’s moisture swell in wood framing or trim. It will not fix itself, but it also will not collapse. Scheduling a fix within 30 days prevents the moisture from becoming rot.
• Efflorescence on foundation walls or slab edges. That white, chalky powder on concrete means water is moving through the concrete and depositing minerals on the surface. It’s a sign of hydrostatic pressure or poor drainage. It won’t destroy the foundation in a month, but it will in a year. Redirect downspouts and regrade soil within 30 days.
• Gaps opening between the ADU and the main house. On attached additions, a gap between the new structure and the existing house means differential settlement. The ADU is settling at a different rate than the original foundation. This is fixable with underpinning or pier adjustment, but only if caught early.
• Increased energy bills without a change in usage. If your PG&E bill climbs 20 percent or more and your habits have not changed, the ADU envelope has a leak. It might be insulation settling, a failed seal around a window, or a duct leak in the crawlspace. The fix is usually under $2,000 in month one and over $6,000 in month twelve, because a leak that persists through a full wet season damages more than just the seal.

Tier 3: Monitor Quarterly

These signs are normal. They appear in nearly every new ADU in San Francisco within the first 18 months. They need watching, not repair. Photograph them once per quarter and compare against the previous photos.

• Hairline cracks under 1/8 inch in drywall corners, especially at window and door corners. This is normal lumber drying and settling. It typically stops within 18 months of completion.
• Minor stucco checking or spider webbing. Fine, crack-like lines on stucco that are not wide enough to fit a fingernail. This is surface shrinkage, not structural.
• Slight nail pops in drywall. Small circles or bumps where drywall nails or screws have backed out slightly. Cosmetic. Fix when repainting.
• Floor squeaks that stay in one location. A single squeak in a consistent spot is usually a subfloor fastener backing out. It’s annoying, not dangerous. If the squeak migrates or new squeaks appear monthly, that’s different, and you should escalate it.

Foundation and Structural Signs: What’s Settling and What’s Failing

Every structure settles. The question is whether the settlement is uniform or differential. Uniform settlement means the whole building drops slightly at the same rate, and that is almost never a problem. Differential settlement means one corner drops faster than the others, and that is where foundations crack, walls shear, and doors jam for good.

In San Francisco specifically, the biggest variable is soil. Much of the city sits on sand, artificial fill, or bay mud. The Marina, Mission Bay, SoMa, and parts of the Richmond and Sunset are built on fill or former marshland. A detached ADU on a reinforced slab on grade behaves very differently than a garage conversion that sits on a 1950s perimeter foundation with no shear wall. Before you interpret any crack, know what your ADU sits on.

How to Distinguish Normal Settling from Structural Movement

1. Measure the crack. Use a simple crack gauge you can buy for under $20, or tape a piece of paper across the crack and mark the date. If the paper tears within 30 days, movement is active. If it stays intact for a full quarter, the crack is dormant.
2. Look at the crack’s shape. A straight, vertical hairline crack is usually cosmetic. A diagonal crack that runs from a window corner toward the floor at a 30 to 60 degree angle means shear stress on the wall. A stair-step crack in a concrete block or brick foundation is movement, full stop.
3. Check both sides of the wall. A crack that appears on the interior drywall and the exterior stucco in the same location means the crack goes through the wall assembly, not just the surface finish. That is structural.
4. Watch the doors. A door that sticks at the top of the frame means the header is dropping. A door that rubs at the latch side means the wall is racking. Take note of which door, which season, and whether it changes.

If you find any of those structural indicators, call a structural engineer before you call a contractor. An engineer’s report costs $800 to $2,500 in San Francisco depending on scope, and it tells every contractor exactly what needs to happen. Without the report, you’re paying someone to guess. With it, you’re paying for a specified repair. The difference is usually $10,000 or more.

We’ve inspected ADUs in the Outer Sunset where homeowners had lived with a “sticky back door” for two years before calling us. The cause was a corner of the foundation settling into saturated sand, dropping 1.4 inches. The fix at two years was helical pier underpinning at $18,000. Had it been addressed when the door first stuck, the fix would have been drainage correction and grout injection under $4,000. This is what we mean when we say delay compounds cost.

If you’re planning construction, work with an ADU builder in San Francisco who understands local soil conditions, not just floor plans. The difference shows up in the foundation details before it ever shows up in a crack.

Moisture and Water Intrusion: The Invisible Damage Multiplier

Moisture is the warning sign you cannot see until it’s expensive. It works behind the drywall, under the flooring, and inside wall cavities for months before it produces anything visible. By the time you see a stain or smell must, the damage is established. What you’re really looking at is the evidence of the damage, not the beginning of it.

San Francisco receives an average of 23 inches of rain per year, but more importantly, the city sits in a marine layer that keeps exterior surfaces damp for long stretches even when it’s not raining. Moisture that gets into a wall cavity during a December storm may not dry until April. That’s four months of continuous damp on untreated framing lumber. That’s how rot starts.

What to Smell

• Earthy, damp smell after rain. This means water is entering somewhere and saturating an organic material, usually wood or drywall. The smell often shows up before any stain.
• Sharp, chemical smell. This can be off-gassing from materials, or it can be mold. Some mold species produce a smell that reads as sharp or acrid rather than musty.
• Sewage smell that comes and goes. Intermittent sewage odor usually means a venting issue or a dry P-trap, but it can also mean a cracked cast-iron or ABS waste line in a slab. Persistent sewage odor means active leak.

What to Feel

• Cold walls in winter. An exterior wall that feels significantly colder than interior walls means insulation has settled, compressed, or was never installed correctly. Cold walls condense moisture from interior air, and condensation inside a wall cavity is a slow, hidden leak.
• Soft spots in flooring. Press around the perimeter of rooms, especially near exterior doors, under windows, and around plumbing fixtures. A floor that gives slightly or feels spongy means the subfloor is wet or rotted.
• Crumbling grout or caulk. Run a finger along bathroom and kitchen grout lines. Loose, sandy, or missing grout means water has been penetrating the seam and working on the backer board behind it.

What to Probe For

• Gaps around window and door flashing. From the exterior, look at the top edge of every window and door. There should be flashing (a metal or membrane lip that directs water outward) and it should extend past the trim, not end at it. Missing or short flashing is the number one source of water intrusion in San Francisco ADUs.
• Grade sloping toward the structure. Soil should slope away from your ADU foundation at least 6 inches over the first 10 feet. If soil slopes toward the foundation, every rain event is delivering water directly to the slab edge or crawlspace.
• Clogged or disconnected downspouts. Check that every downspout connects to a drain or daylight and discharges at least 5 feet from the structure. A downspout that dumps water at the foundation corner is a moisture problem that will show up as a foundation crack within three to five years.

Garage Conversions vs. Detached ADUs vs. Attached Additions: They Fail Differently

Each type of ADU has its own failure profile. Treating them as interchangeable leads to misdiagnosis and wasted money. Here’s what we see most often on each type in San Francisco.

Garage Conversions

Garage conversions in San Francisco are almost always built on existing perimeter foundations that were originally designed for a garage, not living space. The concrete is typically thinner, the reinforcement is lighter, and there is no vapor barrier under the slab. That last point matters most: without a vapor barrier, groundwater wicks up through the concrete and into the flooring above. If you’re converting a garage and the slab does not have a proper moisture barrier, you will see cupping hardwood, bubbling vinyl, or persistent musty odor in the first wet season. The fix requires removing flooring, installing a barrier, and often grinding or replacing the slab. This is a $12,000 to $25,000 fix in San Francisco.

Garage conversions also fail at the transition points: where the new wall meets the old garage door opening, where the new floor meets the driveway apron, and where the new living space meets the original foundation. These are the first places to check for cracks, gaps, and water intrusion.

If you’re planning to convert your garage, a room addition contractor in San Francisco who knows garage conversion specifics will cost more than a handyman, and cost less than the remediation you’ll pay later if the slab wasn’t addressed correctly.

Detached ADUs

Detached ADUs fail most often at the building envelope, because they have four exposed walls instead of sharing one or two with the main house. The most common signs are stucco cracks at window corners, flashing failures around wall penetrations, and roof drainage that concentrates water at one corner of the foundation. Detached ADUs also fail at the utility trench where water, sewer, and electrical lines run from the main house. A settling trench shows up as a long, straight depression in the yard leading from the house to the ADU. If you see that depression, the backfill was not compacted correctly, and the pipes below may be stressed.

Attached Additions

Attached additions fail at the connection point. The new foundation is poured against the existing foundation, and they almost never settle at the same rate. The result is a visible crack at the seam between the original house and the addition, either in the exterior wall, the interior floor, or the ceiling. Some separation is normal in the first year, but a gap that grows past 1/4 inch, or a floor transition that develops a visible change in height, means differential settlement that needs an engineer’s evaluation. Attached additions also fail at the roofline where the new roof ties into the old. Check that tie-in during every rainy season for water staining on the ceiling below it.

Which Signs Void a Builder’s Warranty If Ignored (And How to Document Reporting)

This section matters more than most homeowners expect. Builders write warranties that require the homeowner to report problems promptly. “Promptly” is usually defined as within a reasonable time, and what counts as reasonable depends on what the sign was and when it appeared. Ignore a known problem long enough, and the builder can deny the claim because the delay made the problem worse.

The warranty on your ADU should be specified in your contract. At Sutton ADU San Francisco, the 365-Day Done Right Promise covers defects in workmanship for one full calendar year from project completion. It’s a written guarantee stamped on every project, not a verbal assurance. If a covered element isn’t right within that year, we make it right. There’s no haggling over whether it counts.

Signs That Void or Limit a Warranty Claim If Ignored

• Known water intrusion. If you see a water stain or active leak and wait six months to report it, the rot and mold that developed during those six months is not covered. The original leak might still be covered, but the secondary damage is on you.
• Known electrical symptoms. A flickering circuit or a breaker that trips repeatedly is a hazard. If you ignore it and a fire or equipment failure occurs, the builder is not responsible for damage caused by your failure to disconnect the circuit and report it.
• Foundation or structural signs. A crack that grows from 1/8 inch to 3/4 inch over a year tells the builder you watched a structural problem develop and did nothing. Many warranty exclusions specifically reference “failure to mitigate.”
• Unapproved repairs. If you hire a third-party contractor to fix a warranty item without giving the original builder a chance to inspect and repair, many warranties void coverage for that entire system, not just the repaired part.

How to Document Your Report

1. Photograph the sign with a date stamp. Camera phones embed dates automatically, but take a photo that includes context: the surrounding wall, the room, the fixture. A close-up of a crack tells nothing about where the crack is.
2. Write an email or text to the builder. Verbal reports do not hold up. Send written notice with the photo attached. Reference the date you first noticed the issue and the date of the photo.
3. Keep a simple log. A spreadsheet with four columns: date noticed, description, photo filename, action taken. This takes five minutes per incident and is worth thousands in a warranty dispute.
4. Request a written acknowledgment. If the builder acknowledges your report, ask for a one-line email confirmation. If they do not respond, your sent email is still a dated, written record, which is what matters.

If you are looking for a home addition contractor in San Francisco, ask to see the warranty in writing before you sign. Not a brochure. The actual warranty language. If the contractor will not show you the warranty before you sign the contract, that’s your first warning sign, and it’s one you catch before any work begins.

Electrical and Plumbing Warning Signs Specific to ADUs

ADUs have electrical and plumbing systems that differ from the main house in one important way: they are usually fed by a single subpanel and a single service line from the main structure. That means a problem at the subpanel affects the entire ADU, not just one room. It also means load calculations matter more than they do in a main house.

Electrical Signs

• A subpanel that feels warm to the touch. A properly loaded subpanel should be cool or room temperature. Warmth means a connection is loose, a breaker is overloaded, or the panel is undersized for the connected load. In an ADU with a kitchen, laundry, and heating, an undersized panel is a common and serious problem.
• Breakers that trip more than once per month. One trip after a storm or a power surge is normal. A breaker that trips regularly means the circuit is overloaded or has a short. “Overloaded” in an ADU often means the circuit was originally sized for a garage or a shed and was not upgraded during conversion.
• Lights that dim when a major appliance starts. A brief dim when the heat pump compressor kicks on can be normal inrush current. But lights that dim significantly, or dim on one circuit when an appliance on a different circuit starts, indicate a shared neutral or undersized feeder. This needs an electrician’s diagnostic, not a bulb change.

Plumbing Signs

• Gurgling drains. A drain that gurgles when another fixture runs means the vent is blocked or was never installed correctly. In San Francisco, ADU plumbing is inspected, but a venting defect can pass inspection if it’s marginal and only shows symptoms under specific conditions. Gurgling is the symptom.
• Slow drains in one fixture only. If one sink drains slowly but the rest of the ADU drains fine, it’s a local clog or a slope problem in that branch line. If multiple fixtures drain slowly, it’s the main line serving the ADU, and that’s a different and more expensive problem.
• Water hammer. A loud bang when a faucet or toilet valve closes means the water pressure is too high or the air chambers have failed. In an ADU fed by the main house’s water service, the pressure regulator is set for the main house, which may be too high for the ADU’s smaller pipes. Persistent water hammer can crack copper joints over time.

Most ADU builders in San Francisco include electrical and plumbing in their scope, and the best ones bring the subpanel and main line up to code correctly the first time. If you’re buying a property with an existing ADU, check the subpanel amperage and ask for a plumbing vent test during your inspection. Both are inexpensive checks that reveal expensive problems.

Prefab and Modular ADU-Specific Signs: Cover, Dvele, Mighty Buildings, and Boxabl

Prefab and modular ADUs solve many on-site construction problems, but they introduce their own failure modes. The signs are different because the construction method is different. We work with several major prefab brands, including Cover, Dvele, Mighty Buildings, and Boxabl, so we see these issues specifically.

The most important prefab-specific warning sign is seam separation at module joints. A prefab ADU arrives in modules or panels that are joined on-site. The joints between modules are sealed with gaskets, tapes, and exterior flashing. If those joints shift during transport or installation, the seal can break. The sign is a visible gap, a crack in the interior finish at a module line, or water staining near a module joint. On a Boxabl unit, which folds for transport, the fold lines are the highest-risk locations. On a Dvele or Cover unit, the panel joints are the focus. On a Mighty Buildings unit, the 3D-printed panel seams need watching.

Second, transportation stress cracks in prefab panels are different from site-built settling cracks. A prefab panel that was stressed during trucking or crane placement can crack internally while looking fine on the exterior surface. The sign is usually a crack that appears in the first 90 days, not gradually over 18 months. If you see a crack appear suddenly in a prefab ADU within the first few months, it’s more likely transport or installation stress than settling.

Third, point-load settlement under module corners. A prefab unit sits on a foundation, usually a slab or piers. Each module corner carries a precise load. If the foundation was not poured dead level, or if the soil under one corner compresses, the entire module rotates slightly. The sign is a door that binds at the top of the frame in one module but not another, or a floor transition between modules that develops a lip. Prefab units are less forgiving of foundation tolerance than site-built framing, because the module was built square in a factory and expects the foundation to match.

For homeowners planning a prefab ADU in San Francisco, the foundation tolerance spec matters more than the floor plan. Ask your builder what the manufacturer’s flatness tolerance is, and make sure the foundation contractor commits to it in writing. We see this issue with all four brands we work with, and it’s always cheaper to fix the foundation before the module arrives than to shim a module afterward.

If you’re uncertain which prefab system fits your San Francisco lot, work with a builder who has installed several, not one. Each brand has different foundation requirements, ceiling heights, and exterior systems. An experienced ADU builder in San Francisco can tell you which product line actually fits your lot, your budget, and your timeline, instead of selling you the one they happen to represent.

How San Francisco’s Climate Specifically Attacks ADUs

San Francisco’s climate is unusual for California. It is not the dry heat of Los Angeles, the valley sun of Sacramento, or the freeze cycles of Tahoe. San Francisco is a marine climate with moderate temperatures, high humidity, and a long fog season. That combination attacks buildings in specific ways.

The most aggressive attack is condensation. San Francisco rarely gets hot enough to dry out a wall cavity the way an inland climate does. When moisture gets into a wall in the Outer Richmond or the Sunset, it stays there. The result is a slow, persistent damp that leads to mold and rot even in buildings with no leaks. The moisture source is simply warm interior air meeting cold exterior sheathing and condensing. The fix is not caulk. It’s proper wall assembly: vapor barrier on the correct side, adequate insulation, and a rainscreen behind the cladding. If your ADU was built without a rainscreen and you see paint peeling on the exterior, the moisture is working from the inside out.

The second attack is wind-driven rain. San Francisco’s storms come off the Pacific horizontally, not vertically. Rain hits the west-facing walls at an angle and gets driven into any gap, crack, or flashing defect that a vertical rainstorm would never touch. The west and southwest exposures are the highest risk on any ADU. If you see water staining on a west-facing wall, the leak is almost certainly wind-driven rain finding a flashing gap.

The third attack is marine air corrosion. The salty fog that rolls through the Sunset and the Richmond corrodes metal hardware, flashing, electrical connections, and fasteners faster than inland air. Galvanized hardware lasts 20 to 30 years in most California climates and 12 to 15 years in San Francisco’s fog belt. Stainless steel is worth the upgrade on exterior fasteners and flashing, especially on the west side of the building. If your ADU is near the ocean and you see rust staining on exterior hardware within five years, the hardware was not adequately specified for the environment.

Working with a builder who understands these specific exposures matters more than any design philosophy. The same ADU plan built in the Mission and the Outer Sunset needs different specifications. We see this distinction get missed by builders who build on spec and by homeowners who prioritize the floor plan over the building science. In San Francisco, the building science is the floor plan.

Common Mistakes to Avoid

• Ignoring a sign because “it’s a new build.” Plenty of homeowners assume a new ADU cannot have serious problems and dismiss early signs as nothing. New ADUs fail too. The difference is that new ADUs usually fail from installation errors, which are covered by warranty if reported promptly. Ignoring them doesn’t make them disappear. It makes them uncovered.
• Attempting DIY fixes on structural cracks. Filling a structural crack with epoxy or caulk hides the evidence without fixing the cause. The structure keeps moving, and now you’ve made the sign invisible to the person who could have diagnosed it. For a $12 caulk job, you might cost yourself a $15,000 structural repair that warranty would have covered.
• Comparing cracks in your ADU to cracks in your main house. A 100-year-old San Francisco Victorian has moved all it’s going to move. A two-year-old ADU is still settling. What looks alarming in the ADU might be normal for its age, and what looks normal in the main house might be alarming in a new structure. Age changes the interpretation.
• Waiting until the rainy season ends to fix a leak. San Francisco’s wet season is when you discover leaks and the worst time to fix them, because repairs don’t cure correctly in damp conditions. But waiting until April means the damage gets four more months to compound. The right sequence is: tarp or temporary protection in the wet season, permanent fix as soon as weather allows.
• Not knowing where your water shutoff and subpanel are. If a pipe bursts in your ADU, the first thing you need is the shutoff. It’s usually on the main line where it branches to the ADU. If you don’t know where it is, you’ll spend five minutes finding it while water floods your floor. Know it before you need it, and label it.
• Assuming the builder will catch everything during the warranty walkthrough. Builders catch what is visible during a scheduled walkthrough. Signs that appear three months later are yours to catch. Walk your ADU once per quarter during the first year, specifically looking for the Tier 1 and Tier 2 signs in this guide, and document everything.

When to Call a Professional

Call a professional when you see any Tier 1 sign, any structural indicator from the foundation section, or any moisture sign you cannot source. If you’re not sure whether a crack is active, call. If you have a water stain and cannot find the leak, call. If the smell of gas or sewage appears, call immediately. The cost of a diagnostic visit is rarely more than $200 to $400, and it either gives you certainty or finds a problem while it’s still cheap to fix.

When it’s time to plan the fix, or if you’re still in the planning stage and want to avoid the warning signs altogether, the quality of the original build determines how many of these signs you’ll ever see. Sutton ADU San Francisco is the ADU-specialist member of the Threshold Group, founded in 2006. We offer free estimates in San Francisco, a written price before any work begins, and the 365-Day Done Right Promise on every project. Call (831) 272-7212 to schedule yours.

Frequently Asked Questions

The Bottom Line

Most ADU warning signs in San Francisco are not emergencies, but a handful are, and knowing which is which is the difference between a