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

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

Here’s a number that surprises most San Francisco homeowners: a small water stain on an ADU ceiling, left alone for six months, typically costs three to four times more to repair than if you’d caught it in the first two weeks. Moisture moves fast behind drywall. A hairline crack above a door frame and a hairline crack along a foundation stem wall can look nearly identical to the untrained eye, but one is cosmetic paint movement and the other is a conversation you need to have with a structural engineer this month. This guide teaches you to read your accessory dwelling unit, whether it’s a garage conversion in the Sunset or a backyard cottage in Bernal Heights, the way a contractor reads it: symptom by symptom, cause by cause, and with an honest urgency rating attached to each. For a deeper dive into the full ADU process, see The Complete Guide to ADU in San Francisco. You’ll learn what to photograph, what to measure, what to ignore, and what should have you on the phone before your next cup of coffee.

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Quick Answer

The most common ADU warning signs in San Francisco fall into four categories: moisture intrusion (staining, bubbling paint, musty smells), structural movement (new or widening cracks, sticking doors), electrical strain (flickering lights, warm outlets, tripped breakers), and drainage failure (standing water, downspout discharge too close to the foundation). A slow-growing vertical hairline crack is usually cosmetic; a horizontal crack wider than 1/8 inch or any crack that grows noticeably within one rainy season demands professional evaluation. When in doubt, document the sign with photos and measurements, then call a contractor who will give you a written assessment, like Lodgewick ADU San Francisco at (707) 760-0502. For more on deciding between tackling issues yourself or hiring help, read our DIY vs Professional ADU: The San Francisco Homeowner’s Decision Guide.

Table of Contents

• Crack Taxonomy: Reading the Lines in Your Walls and Foundation
• Moisture Warning Signs in Shared-Wall ADUs
• Electrical Red Flags in San Francisco’s Aging Housing Stock
• Roof Issues: Ridge, Flashing, or the Field
• The Documentation Process: How to Photograph and Describe Warning Signs
• What Early vs. Late Intervention Actually Costs
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions

Crack Taxonomy: Reading the Lines in Your Walls and Foundation

Most San Francisco homeowners see a crack and immediately think foundation failure. That’s understandable, but it’s also wrong more often than not. The city’s clay-heavy soil, especially in districts like the Richmond, Sunset, and Excelsior, expands and contracts with seasonal moisture shifts. That movement causes cosmetic cracking in drywall and stucco, hairline lines that open and close with the weather but never threaten the structure. The key is learning which cracks are talking and which ones are shouting.

Vertical hairline cracks, under 1/16 inch wide, running up from a door or window corner, are almost always cosmetic. They appear when framing lumber settles or when drywall compound shrinks after installation. In an ADU built within the last three years, these are expected, not alarming. Paint over them and monitor. If they return wider than before, that’s a different conversation.

Diagonal cracks radiating from a corner at roughly 30 to 45 degrees typically indicate differential settling: one part of the structure is moving slightly more than the part next to it. In San Francisco ADUs built over pier-and-post foundations, common in older garage conversions, diagonal cracks near the top of a wall may point to a beam flexing under load. A 1/16-inch diagonal crack you can’t fit a fingernail into is worth photographing and checking monthly. If it widens to 1/8 inch within a season, call a structural professional.

Horizontal cracks are the ones that matter most. A horizontal crack running along a foundation stem wall, or along a retaining wall holding back a sloped San Francisco lot, signals lateral pressure: the soil outside is pushing inward. This is not a paint problem. In neighborhoods like Glen Park, Diamond Heights, and Twin Peaks, where homes step down steep grades, lateral soil pressure is a constant force. A horizontal crack wider than 1/8 inch, or one that lets you see daylight or feel a draft, is a professional consult this month condition. The same applies to stair-step cracks in brick or block foundation walls: the mortar joints are failing in a pattern that maps directly to differential settlement under the footing.

One more San Francisco-specific note: if you live in a liquefaction zone, which covers much of the Marina, Mission Bay, and South of Market, any new crack in an ADU foundation warrants a faster response than the same crack in a bedrock-anchored neighborhood like Twin Peaks. Soil type changes the urgency math.

Moisture Warning Signs in Shared-Wall ADUs

ADUs built against an existing home, whether as a garage conversion or a rear addition, create a specific moisture dynamic: the shared wall is no longer an exterior wall for either structure, but the waterproofing behind that wall may not have been upgraded to match. This is one of the most common things we see in San Francisco ADUs built between 2010 and 2020, when permitting rules loosened and some builders moved faster than their flashing details did.

Efflorescence, the white chalky residue that forms on concrete or masonry, tells you water is moving through the wall and depositing mineral salts on the surface as it evaporates. It’s not mold, but it’s the early warning that mold conditions exist behind it. On a foundation wall or a concrete retaining wall, light efflorescence after a wet winter may be acceptable. Heavy efflorescence that reappears after cleaning, or that appears on interior drywall, means there’s an active moisture path.

Bubbling or peeling paint on a shared wall, particularly near the floor line, points to water migrating up from the sill plate or in from the exterior grade. San Francisco fog is a unique contributor here: moisture doesn’t always arrive as rain. Summer fog in the Sunset and Richmond can condense on cool wall surfaces for hours every morning, and if the wall assembly doesn’t breathe correctly, that condensation works its way behind the paint film. When homeowners tell us the paint bubbled in August, when there was no rain, fog condensation is often the hidden culprit.

Musty smell geography is a diagnostic tool most homeowners never use. When you walk into your ADU and smell must, don’t just note “it smells” and move on. Walk the perimeter. Smell high, at the ceiling line, and low, at the baseboard. Smell near plumbing fixtures separately from drywall surfaces. A musty smell that’s strongest near the shower drain points to a plumbing leak or a failed pan. A musty smell that’s strongest at the bottom of a shared wall points to sill-plate moisture. A musty smell strongest at the ceiling under the roof eave points to flashing failure. Telling a contractor where the smell lives, not just that it exists, saves a site visit and often a wall demo.

In San Francisco’s older housing stock, where many ADUs are converted from garages built in the 1920s through 1940s, original slab-on-grade floors often lack a vapor barrier. Moisture rises through the concrete, dampens the bottom plate of walls, and creates persistent low-level must that no amount of air freshener fixes. If your ADU sits over an old garage slab and smells damp year-round, the fix is usually a vapor barrier retrofit, not a dehumidifier.

Electrical Red Flags in San Francisco’s Aging Housing Stock

Adding an ADU to a San Francisco home means adding a second kitchen, a second bathroom, and often a second laundry setup, all drawing power from a panel that may have been sized for a 1940s household with a radio and a single light bulb per room. Electrical warning signs in an ADU are not just about the ADU itself; they’re about the shared service equipment behind it.

Flickering lights under load is the most common early symptom. When your ADU’s lights dim briefly when the microwave kicks on, or when the washing machine starts its spin cycle, you’re seeing voltage drop across an undersized or high-resistance connection. A one-time flicker on startup is normal induction load. Repeated flicker every time a major appliance cycles, especially if it also affects lights in the main house, signals a service sizing issue or a loose neutral connection. Both are fixable, but neither fixes themselves.

Warm outlets or switch plates are never normal. A warm plate on a circuit that’s not running a space heater or other high-draw device means resistance somewhere in that outlet’s wiring, usually a loose terminal screw or a worn receptacle. Left alone, this is how electrical fires start in older San Francisco homes with original cloth-insulated wiring. If the plate is too warm to comfortably hold your palm against for ten seconds, turn off that breaker and call a licensed electrician that day. This is not a “schedule it next week” item.

Tripped AFCI breakers, especially those that started tripping after the ADU was added, often indicate arc faults: small electrical arcs caused by damaged insulation, loose connections, or pinched wires. The AFCI is doing its job. The mistake homeowners make is resetting the breaker repeatedly while ignoring what it’s trying to tell them. If a specific AFCI breaker trips more than twice in a month, the circuit needs diagnosis, not a stronger reset finger. In our experience across San Francisco ADU projects, especially those built before 2020 when AFCI requirements expanded, nuisance tripping on shared circuits often traces back to a neutral wire shared between the main house and the ADU subpanel, a violation of current code that’s inexpensive to identify and correct.

If your ADU shares a panel with the main house, which most San Francisco ADUs do, pay attention to whether the main house’s electrical behavior changes after the ADU is built. Breakers that never tripped before but now trip on hot days, or a main breaker that feels warm to the touch, point to a service capacity issue. A 60-amp or 100-amp service that was fine for one household may be drowning under two.

Roof Issues: Ridge, Flashing, or the Field

Roof leaks in San Francisco ADUs tend to fall into three categories, and each has its own urgency and cost profile. Learning to identify which category you’re looking at, even roughly, can save you thousands of dollars and months of hidden water damage.

The ridge is the peak where two roof planes meet. Ridge leaks in San Francisco are wind-driven more often than rain-driven: the city’s westerly winds push rain up over the ridge cap, and if the ridge cap’s sealant is old or cracked, water gets under the cap and travels down the underside of the decking before it drips. This means the drip you see inside may be ten or fifteen feet downhill from the actual entry point. Ridge leaks caught early require re-sealing or replacing the ridge cap, a $300 to $800 repair on most ADU roofs. Missed for a season, the same leak rots decking and rafters, and the repair climbs into the $2,500 to $6,000 range.

Flashing is the metal that seals the roof around penetrations: skylights, vents, plumbing stacks, and the wall line where the ADU roof ties into the main house. Flashing failures are the most common leak source in San Francisco ADUs, particularly at the roof-to-wall junction on shared-wall builds. The reason is simple: a roof that originally shed water freely as a garage roof is now tied into a taller wall, and the flashing detail at that junction has to handle both direct rain and the concentrated run-off from the main house’s wall above. When a San Francisco homeowner tells us they see water staining on the ceiling at the point where the ADU meets the main house, flashing is our first suspect. Early flashing repair: $500 to $1,500. Late flashing failure with structural damage to the header or sill behind it: $4,000 and up.

The field is the broad open area of the roof between the ridge and the eaves. Field leaks are usually material failures: cracked composition shingles, lifted edges on rolled roofing, punctures from falling eucalyptus branches, or failed patches over old penetrations. On flat or low-slope roofs, common on San Francisco ADUs with modern designs, field leaks often originate at ponding areas where water sits for more than 48 hours after rain. If you can safely see your ADU roof from a window, photograph it after the next rain and look for areas that stay shiny-wet while the rest dries. Those ponding zones are where your next leak will start. Flat-roof field repair runs $400 to $1,200 caught early. The same leak that sat through a full San Francisco winter, sending water through the insulation layer and into the ceiling below, runs $3,000 to $8,000 when you factor in decking replacement and interior repair.

A note on safety: roof inspection from a ladder is genuinely dangerous work. More homeowners are injured falling from ladders every year than are injured by the roof problems they’re trying to inspect. If you can’t see the roof from a window or a safely accessible adjacent deck, have a professional look at it.

The Documentation Process: How to Photograph and Describe Warning Signs

A well-documented warning sign can save you a site visit charge, a wrong diagnosis, and weeks of waiting. A poorly documented one can send a contractor down the wrong path entirely. Here’s the process we recommend to every San Francisco homeowner who calls us about a concern: document first, call second.

Step 1: Photograph with context and detail. Take two photos. The first is a wide shot showing the warning sign in relation to the room, the wall, or the roof. This gives the contractor location context. The second is a close-up with something for scale: a quarter, a pencil, or a tape measure held against the crack or stain. A photo of a crack with no scale tells a contractor nothing about urgency. A photo of a crack with a quarter next to it, showing the crack is wider than the quarter’s thickness, says everything.

Step 2: Date and measure. Note the date of the photo and the dimensions of the sign. For a crack: width in inches or millimeters. For a stain: approximate diameter. For a water spot on the ceiling: diameter and whether it feels damp to the touch, dry, or crusty. Keep a simple note on your phone or in a notebook. “Crack above ADU bathroom door, July 12, 1/16 inch wide, vertical.” Six months later, when you check again, you’ll have a baseline. Without a baseline, you cannot know if the crack is growing.

Step 3: Describe conditions at the time. Was it raining? Had it rained in the previous 48 hours? Was the heat running? Was the washer going? Were you running a space heater? These details connect the symptom to the cause. A ceiling stain that appears only after heavy rain points to the roof. A ceiling stain that appears regardless of weather points to a plumbing line. The same stain, described to the same contractor, leads to two completely different diagnoses depending on the conditions you report.

Step 4: Check for patterns. Look at the sign over the course of a week. Does the crack open and close? Does the stain change size? Does the flickering happen only at night, only when it’s windy, only when the oven is on? Patterns are diagnostic gold. A homeowner who can tell us “the lights only flicker when the dryer runs and only on rainy days” has already narrowed the problem from “something electrical” to “probably a compromised exterior connection or a neutral issue at the weatherhead.”

Step 5: Send it all in one message. When you call or email a contractor, include the photos, the measurements, the date, and the conditions. This package lets an honest contractor give you a meaningful preliminary assessment without a site visit, which saves you money and helps you decide whether you need a full inspection or just a watch-and-wait plan. At Lodgewick, we take these calls every day, and the homeowners who show up with documentation get better answers faster.

What Early vs. Late Intervention Actually Costs

Here’s the honest math, based on San Francisco labor and material rates in 2025 and early 2026, for the warning signs covered in this guide. These ranges assume a typical ADU footprint of 400 to 800 square feet. Larger or more complex structures can run higher.

• Vertical hairline crack, caught early: $150 to $400 for drywall repair and repaint. Caught late, after it widens into a structural issue: $2,000 to $8,000 for foundation assessment and repair.
• Horizontal foundation crack, caught early: $600 to $1,500 for carbon fiber reinforcement or epoxy injection. Caught late, after wall movement becomes visible above grade: $15,000 to $40,000 for foundation replacement or underpinning, plus interior repairs.
• Efflorescence on a shared wall, caught early: $200 to $600 for surface remediation and sealing. Caught late, after framing damage: $3,000 to $10,000 for wall demo, sill plate replacement, and re-sheathing.
• Bubbling paint from fog condensation, caught early: $300 to $800 for ventilation correction and spot repair. Caught late, after mold sets into the wall cavity: $2,000 to $7,000 for mold remediation plus rebuild.
• Warm outlet, caught early: $150 to $350 for outlet replacement and connection tightening. Caught late, after the wiring insulation fails: $2,000 to $5,000 for circuit replacement, plus fire damage risk that can reach six figures.
• Flashing leak, caught early: $500 to $1,500 for re-flashing. Caught late, after header and sill damage: $4,000 to $12,000 for structural repair plus interior restoration.
• Field roof leak, caught early: $400 to $1,200 for patch or section replacement. Caught late, after decking rot: $3,000 to $8,000 including insulation replacement and ceiling repair.

Common Mistakes to Avoid

• Painting over a recurring crack without measuring it first. Paint hides the evidence and destroys your baseline. If the crack comes back, you’ve lost months of diagnostic history. Measure and photograph before you touch anything.
• Ignoring a musty smell because there’s no visible mold. By the time mold is visible on the surface, it’s been growing inside the wall for weeks or months. Musty smell is the early warning system, and it’s free. Use it.
• Resetting a tripped AFCI breaker repeatedly. The breaker is telling you something. The second or third reset in a month is the electrical system asking for help. Ignoring it is how arc faults become fires.
• Assuming a small roof leak is contained to the ceiling it stains. Water travels. A leak at the roof-to-wall junction in your ADU can drip inside the ceiling, run along a joist, and damage a wall ten feet away before it ever stains drywall. The stain you see is rarely the only damage.
• Waiting until summer to fix a winter leak. San Francisco’s rainy season runs roughly November through March, and the fog season covers the rest. There is no reliable dry window to “wait for.” Address moisture issues when you find them, not when the calendar looks convenient.
• Hiring a generalist for a diagnosis without documentation. Send your photos and notes before booking. A contractor who sees your documentation and still can’t give you a preliminary read is either too busy to care or not experienced enough to help.
• Trusting a verbal repair promise. If a contractor won’t put the diagnosis and the repair scope in writing, walk away. Written commitments are the floor, not the ceiling, for any work on a San Francisco ADU.

When to Call a Professional

Call a professional when a horizontal crack is wider than 1/8 inch, when any crack grows measurably within a season, when an outlet or switch plate is too warm to hold comfortably, when a breaker trips more than twice in a month, when a ceiling stain reappears after repair, or when a musty smell persists despite cleaning and ventilation. These are not watch-and-wait conditions. These are call-this-week conditions.

Lodgewick ADU San Francisco offers free estimates throughout San Francisco, and every estimate includes a written price before any work begins. If you’ve documented a warning sign in your ADU and want an honest read on urgency, call (707) 760-0502. A live person answers, day or night.

Frequently Asked Questions