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

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

A hairline crack running diagonally from the corner of an ADU window frame in San Antonio isn’t necessarily a foundation problem – but the direction it runs and whether it’s wider at the top or bottom tells you which of three very different causes is actually at work. Most accessory dwelling units fail quietly. They crack, stain, smell, or sag for weeks before a repair becomes expensive, and the owners who catch those early signals are the ones who spend hundreds instead of thousands. This guide walks you through every warning sign that matters for a San Antonio ADU, what each one actually means, and which of them demand action within 30 days. For a deeper dive into regulations and planning, see The Complete Guide to ADU in San Antonio.

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Quick Answer

The warning signs that matter most in a San Antonio ADU are diagonal cracks wider at the top than the bottom, doors or windows that bind seasonally, musty odors near the floor line, visible roof sag or lifted shingles, and warm outlet covers or flickering lights under load. Most of these can be measured or observed in under ten minutes without special tools. If you spot any of the structural or electrical signs below, call a contractor for a professional inspection before the next heavy rain or heat wave. For help choosing who to hire, read our DIY vs Professional ADU: The San Antonio Homeowner’s Decision Guide.

Table of Contents

• Cracks in Stucco, Drywall, and Foundation: What They Actually Mean
• Musty Smells in a Detached ADU: The Two Spots to Check First
• Doors and Windows That Bind: Seasonal Shift vs. Foundation Movement
• Roof Warning Signs Visible From the Ground
• Electrical Warning Signs in Small Structures
• Water Intrusion and Drainage Patterns Around the ADU
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions
• The Bottom Line

Cracks in Stucco, Drywall, and Foundation: What They Actually Mean

Not every crack in a San Antonio ADU means trouble. The city’s clay-heavy soils expand and contract through wet-dry cycles that run hard from spring storms to August drought, and small surface cracks in stucco are a normal part of that movement. What separates a cosmetic crack from a structural one comes down to three measurements: width, direction, and whether one side has shifted relative to the other.

Here is the at-home measurement method we teach San Antonio ADU owners, and it takes about five minutes:

1. Tape a strip of painter’s tape across the crack, marking the date on the tape with a pen.
2. Measure the crack’s width at the top, middle, and bottom with a ruler. Write those three numbers down.
3. Wait 60 days and measure again at the same three points.
4. If the crack widened by more than 1/16 inch in that window, or if one side of the crack now sits proud of the other, the movement is active and structural.

Direction matters as much as width. Vertical cracks in San Antonio ADU walls are usually the least concerning, often nothing more than drywall seam separation from humidity swings. Horizontal cracks along a foundation stem wall are the ones that deserve immediate attention, because they suggest lateral soil pressure pushing the wall inward. Diagonal cracks that start at a window or door corner and run outward are the classic sign of differential settlement: one side of the opening is sinking faster than the other, concentrating stress at the frame corner.

Here is the pattern we see across San Antonio neighborhoods like Alamo Heights, Stone Oak, and Helotes: ADUs built on pier-and-beam foundations crack seasonally with little long-term concern, while slab-on-grade ADUs on the city’s western clay bands tend to crack once, significantly, after the first full wet-dry cycle. If your ADU sits on the same slab as the main house, check whether the main house shows the same pattern. Matching cracks in both structures usually point to soil movement rather than an ADU-specific failure. Cracks that appear only in the ADU, especially if it was permitted and poured separately, point to that slab’s specific soil prep.

One more San Antonio-specific note: the 2021 winter storm and the deep freezes that have followed it put real stress on exterior stucco, particularly on the north and east faces of ADUs where morning ice lingers longest. Hairline stucco cracks that opened after a freeze are typically superficial, but they can become water entry points. Seal any exterior crack wider than 1/16 inch before the next rainy season, and if a stucco crack is wide enough to fit the edge of a quarter, have the wall inspected.

Musty Smells in a Detached ADU: The Two Spots to Check First

A musty smell in a detached ADU almost always means moisture is entering somewhere it should not, and in a San Antonio climate the diagnosis follows a predictable order. The smell is not the problem. It is the evidence of the problem, and the two spots to check before calling anyone are the bottom plates of the exterior walls and the underside of the roof deck at the eaves.

Bottom plates are the horizontal framing members that sit directly on the foundation. In many San Antonio ADUs we inspect, the baseboard looks fine but the bottom plate behind it has been wicking water from a sprinkler head that overshoots, a downspout that dumps gutter water two feet from the wall, or a grade that slopes toward the ADU instead of away. Run your hand along the baseboard at the wall’s lowest corner after the next rain. If the drywall feels cool or soft, or if the baseboard has pulled away from the wall by even a quarter inch, water is moving through the wall assembly. That is the first spot.

The second spot is the underside of the roof deck, visible from inside the ADU’s attic access if the unit has one, or from the exterior by looking at the eave line. San Antonio downpours are short and violent, and wind-driven rain enters at eave gaps, gable vents, and where the ADU roof ties into the main house. A musty smell that gets stronger on humid mornings usually traces to the roof plane, not the floor. In detached ADUs without attics, check the ceiling drywall directly below the low edge of the roof for faint yellow rings or paint that has begun to bubble.

Here is the thing about musty smells in San Antonio specifically: the city’s humidity masquerades as the source of the problem all summer long. Homeowners tell us their ADU “smells a little closed up,” so they open windows and run a fan, and the odor fades until the next rain. That pattern is the tell. Seasonal mustiness that follows rain events is water intrusion, not stale air. Persistent mustiness that never entirely goes away, even with ventilation, is usually a slow plumbing leak inside a wall cavity, most often at a toilet wax ring or a shower drain in the ADU’s wet wall. If the smell concentrates within five feet of the bathroom wall, shut off the water supply to the ADU overnight. If the smell is reduced by morning, schedule a leak search.

Doors and Windows That Bind: Seasonal Shift vs. Foundation Movement

San Antonio’s clay soil moves with water, and it moves enough to make interior doors stick in nearly every home in Bexar County at some point during the year. The question is not whether a door binds, but whether it binds on a schedule you can predict, and whether that schedule matches the seasons. A door that sticks every July and swings free every January is almost always a seasonal expansion issue, not a foundation failure.

Here is how to tell the difference in under two minutes:

1. Note which month the binding started and whether it has happened before.
2. Climb under the ADU if it sits on a pier-and-beam foundation and look for a gap between the beam and the pier. A pier that no longer touches its beam is carrying no weight, and the floor above it is sagging.
3. If the ADU is on a slab, measure the gap at the top of the binding door versus the bottom. A door that rubs at the top corner on the latch side means the header is sagging or the slab is dropping at that corner. A door that rubs along the floor means the hinge-side floor has risen, which in San Antonio is almost always clay soil swelling after heavy rain.
4. Open and close the door once a week for a month and mark whether the resistance changes. Foundation movement is progressive. Seasonal movement is cyclical.

The foundation explanation is more common in San Antonio ADUs than in the main house for one simple reason: many ADU slabs are smaller pours than the house, with less reinforcement and sometimes fewer soil prep steps, which makes them more responsive to soil movement. A 400-square-foot ADU slab can deflect noticeably while the 2,000-square-foot main slab stays flat. If only the ADU doors bind and the main house stays true, pay attention. That asymmetry points to something specific to the ADU’s construction.

Window binding matters too, and in a vinyl-window ADU it often shows up as a sash that suddenly requires shoulder pressure to move. Vinyl expands measurably in San Antonio heat, so a window that binds only on afternoons above 100 degrees is behaving normally. But a window that binds at eight in the morning in April is not a heat issue. That is frame racking, and it means the rough opening is no longer square. A window that racks far enough will crack the glass along the lower corner, which is why a diagonal crack from a window corner deserves more attention than one from a drywall seam.

For homeowners considering a new ADU or a Garage Conversion Contractor in San Antonio, soil testing before the slab pour is the single most effective step in preventing binding doors five years later. It costs less than the repair for a settled corner slab, and it tells your builder exactly how deep and how wide the footings need to be for the specific clay your lot sits on.

Roof Warning Signs Visible From the Ground

Most ADU roof failures in San Antonio give you thirty to sixty days of visible warning before a leak becomes a ceiling replacement. The signs are visible from the ground with a pair of binoculars, and they are consistent across shingle, metal, and flat TPO roofs. What they are not is optional to monitor.

From the ground, look for these five things, in this order:

• Lifted or missing shingles along the eave edges. San Antonio’s spring wind storms peel shingles at the rake and eave first, and every exposed nail is a water entry point.
• Shingle granules collecting in the gutters. Granule loss is normal for a roof nearing the end of its life. If you see mineral-colored grit in the downspout splash blocks after every rain, the shingles are losing their UV protection and will begin to curl within a season.
• Visible sag between rafters. Sight down each roof plane from a corner of the lot. A roof plane should read as one flat line. A dip between rafters means the decking is saturated and delaminating, and that decking usually fails within the next heavy storm.
• Rust streaks below roof penetrations. On metal roofs this is how fastener failure announces itself. A rust stain running down from a screw head means the rubber washer under the head has hardened and cracked, and water is traveling down the shank.
• Caulking that has pulled away from vent pipes and skylight curbs. San Antonio’s dry heat cycles harden exterior caulk in about five years. When you can see daylight between the caulk and the pipe flashing, the seal has failed.

Flat roofs on modern prefab ADUs, including models from Cover and Dvele that are designed for California and Texas climates, rely on a membrane system rather than shingles. For those, the ground-level warning sign to watch is ponding water. After a rain, use binoculars to check whether any low spots on the roof still hold water an hour after the rain stops. Water that ponds for more than 48 hours degrades most TPO and modified bitumen membranes at a rate that cuts years off the roof’s service life. A slight pond is not an emergency, but a pond that never drains is worth a professional look before summer’s UV does the damage for you.

If you see any of the five shingle-roof signs above, schedule a professional inspection within 30 days. A roof leak that reaches the ceiling of an unoccupied ADU can go unnoticed for weeks, which is exactly how a $400 repair becomes a $6,000 interior remediation.

Electrical Warning Signs in Small Structures

ADUs have a common electrical design constraint that main houses do not: they are almost always fed by a single 100-amp or 60-amp subpanel run from the main house, and that entire structure’s electrical demand travels through one feeder cable. When an ADU was built with a 60-amp feed and later gains a mini-split, a tankless water heater, and a kitchenette, the system begins to show strain in specific, predictable ways.

The warning signs, in order of concern:

• Flickering lights when a specific appliance cycles on. This is a voltage drop, and it means the appliance’s starting draw is pulling the subpanel voltage below where it should be. In an ADU it most commonly happens with a mini-split compressor or a microwave. A brief dim is normal. A pronounced flicker is not.
• Warm outlet covers. No outlet cover should ever feel warm to the touch. Warmth means resistance heat at the connection, and resistance heat is how electrical fires start. This is the one sign on this list that you should treat as urgent the day you notice it.
• A breaker that trips more than twice. One trip is a fluke. A second trip on the same circuit is a diagnostic message: the circuit is overloaded or there is a loose connection. ADUs that were wired as a garage or workshop and converted later rarely have enough circuits for modern kitchen loads, and breakers on overloaded circuits trip reliably under repeated load.
• Lights that are brighter in the ADU than in the main house. This can indicate a neutral problem at the subpanel, and it is more common than most homeowners realize. A loose neutral sends voltage to the wrong rail, overdriving one leg of the system while underdriving the other.

We want to be direct here: diagnosing electrical problems inside a subpanel is not a DIY project. High-tension and high-current work carries real risk of arc flash and serious injury, and the National Electrical Code exists for a reason. What you can safely do is the observation work above, and then turn off the affected breaker and call a licensed electrician. The observation tells the electrician what to look for, which shortens the diagnostic visit and the bill.

San Antonio homeowners adding an ADU through a ADU Builders in San Antonio team should ask one specific question before signing a contract: what is the subpanel ampacity, and what was the load calculation? A proper load calculation accounts for everything the ADU might reasonably run at once. Prefab builders like Boxabl and Mighty Buildings ship units with known electrical specs, which makes answering that question easier, but site-built and garage-conversion ADUs need the calculation run by a licensed electrician before the walls go up. Once the drywall is in, upgrading a subpanel feed costs triple what sizing it correctly at rough-in would have cost.

Water Intrusion and Drainage Patterns Around the ADU

San Antonio’s rain comes in two modes: long spring soaks and short, hard summer storms that drop an inch in twenty minutes. Both stress the soil around an ADU, and both reveal drainage problems quickly if you know where to look.

Walk the ADU perimeter after the next heavy rain and check these six things:

1. The grade within three feet of the foundation. The ground should slope away from the ADU at about one inch per foot. If water sits against the slab, it will eventually find the cold joint between slab and wall, and a musty interior follows.
2. Downspout discharge points. A downspout that exits within two feet of the ADU wall is dumping water at the foundation. Extend it or add a splash block that directs flow at least six feet out.
3. Sprinkler head spray patterns. Heads that spray directly onto an exterior wall are doing that wall’s bottom plate no favors, especially on the west side where afternoon sun bakes wet framing through the siding.
4. Mud lines on the exterior. A horizontal dirt line partway up the stucco or siding tells you exactly how high surface water reaches during storms. That line is the grade level the water wants to sit at.
5. Expansion joint conditions. Where the ADU meets a patio or sidewalk, look at the caulking in the expansion joint. When it cracks and pulls, water travels straight down into the subgrade beside the slab.
6. Interior floor dampness near exterior doors. A concrete floor that stays darker near the door threshold for hours after rain means the sill is leaking and water is moving under the door assembly.

For those in the planning stage, a Home Addition Contractor in San Antonio can address drainage before a single wall goes up, which is immeasurably easier than regrading around a finished ADU. Swales, French drains, and properly pitched slabs are design decisions, not afterthoughts.

Common Mistakes to Avoid

• Ignoring a crack because it is small. Width matters less than change over time. A 1/16-inch crack that grows to 1/8 inch in sixty days is more concerning than a stable 1/4-inch crack. Measure first, assume later.
• Caulking over a moving crack without figuring out why it moves. Caulk seals weather; it does not stop settlement. The crack will return, usually wider, and now it has a false sense of repair around it.
• Running a dehumidifier as the permanent fix for a musty smell. The dehumidifier treats the symptom. The moisture source is still feeding the wall, and while you dehumidify one room, the framing inside the wall keeps rotting.
• Assuming a new roof means no worries. San Antonio roof failures in the first three years are almost always installation failures: underdriven fasteners, bad flashing at penetrations, or wrong underlayment for this climate. Age is not the only failure mode.
• Ignoring small electrical signs because the breaker resets. A breaker that resets without tripping again feels fixed. It is not. The overload condition is still there, waiting for the next time the floor fan and the hair dryer run at once.
• Waiting until a tenant complains before inspecting. Many San Antonio ADUs house family members or renters, and renters are often hesitant to report problems. A monthly walkaround of the unit, even when occupied, catches more than any tenant report ever will.

When to Call a Professional

Call a professional when you see a diagonal crack widening beyond 1/16 inch in sixty days, a horizontal foundation crack of any width, a warm outlet cover, a roof sag visible from the ground, or a musty smell that persists after ventilation and rain-window checks. These are not wait-and-see items. Each one points to a failure that compounds monthly, and the cost curve is steep.

In our experience building and inspecting ADUs across San Antonio, the homeowners who call early spend a fraction of what the same repair costs after a season of ignoring it. Seedling ADU San Antonio offers free estimates in San Antonio. Call (210) 981-6358 and tell us what you are seeing. We will tell you straight whether it is urgent, and if it is something we do not handle, we will tell you who does.

Frequently Asked Questions

The Bottom Line

An ADU in San Antonio talks to you. It talks through the crack that widens, the door that binds, the outlet that warms, and the musty smell that returns after every rain. The homeowner’s job is not to fix everything on this list; it is to know which signs are seasonal noise and which are structural signal, and to act before the signal becomes loud enough that everyone in the house can hear it. Learn the measurement methods, check the two musty-smell sources first, respect the danger in electrical signs, and walk the perimeter every season. When a sign crosses from watch to act, call (210) 981-6358 for a free estimate, and we will help you make the call with clear information and written pricing before any work begins.

Written by Ben Ortega, Owner at Seedling ADU San Antonio, serving San Antonio since 2001.