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

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

A hairline crack in an ADU slab in San Antonio is either totally normal shrinkage or the beginning of a $15,000 foundation repair, and the difference between those two outcomes comes down to which direction the crack runs and whether it grew since last month. Most homeowners don’t know which question to ask. This guide walks you through the warning signs that actually matter on an accessory dwelling unit in San Antonio, from slab cracks and mini-split behavior to paint bubbling and tenant complaints that sound cosmetic but aren’t. You’ll learn to read your ADU the way a builder does, with a clear threshold for every symptom: watch it, or call someone today.

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Quick Answer

ADU warning signs in San Antonio fall into five categories: slab cracks wider than 1/8 inch or showing vertical displacement, mini-split systems that gurgle or ice up during summer operation, paint bubbling on fiber cement or wood trim that points to water intrusion, electrical symptoms like a GFCI that trips every morning on a shared panel, and tenant complaints about smells or sticking doors that translate into moisture or foundation problems. A hairline crack that hasn’t moved in a year is rarely urgent, but a diagonal crack wider than a nickel or a door that sticks only in July deserves a professional look within the week.

Table of Contents

• Slab Cracks: Reading the Direction and Width
• Mini-Split Warning Signs San Antonio Homeowners Miss
• Exterior Envelope Red Flags in San Antonio Humidity
• Electrical Warning Signs When an ADU Shares a Panel
• Tenant Complaints That Are Actually Structural Warnings
• Sticking Doors and Windows: Seasonal or Structural?
• Common Mistakes to Avoid
• When to Call a Professional

Slab Cracks: Reading the Direction and Width

San Antonio sits on expansive clay soil, which means your ADU’s slab moves every single year. The clay under your foundation swells when it rains and shrinks during drought, and that movement creates stress on the concrete. Not all cracks are created equal, and in our experience, the homeowners who can classify a crack correctly are the ones who avoid paying for a foundation repair they don’t need, or worse, ignoring one they do.

Here’s the classification system we use when we walk a slab in San Antonio:

• Hairline shrinkage cracks (under 1/16 inch wide): These form within the first year as concrete cures and loses moisture. They’re random in direction, don’t run edge to edge, and don’t change with the seasons. Watch them, photograph them, and measure them once a quarter. That’s it.
• Hairline cracks that open and close seasonally (1/16 to 1/8 inch): Common on San Antonio’s heavier clay soils. The crack widens in dry summers and tightens after rain. If the movement stays under 1/8 inch and both sides of the crack remain level, this is the slab doing its job. Still, track it with dated photos.
• Diagonal cracks running corner to corner (1/8 inch or wider): This is the pattern that warrants a foundation engineer. Diagonal cracking usually means differential settlement, the slab settling more in one corner than another. A diagonal crack wider than a nickel, or one that has grown visibly in 90 days, is a call-this-week situation.
• Step cracks in masonry veneer or stem walls: If the cracking follows mortar joints in a stair-step pattern, water is finding a path in, and the movement is structural, not cosmetic. Step cracks on a stem wall below the sill plate are especially worth attention in San Antonio because our clay soil expands laterally as well as vertically.
• Horizontal cracks with one side raised: If you can place a quarter on one side of a crack and it sits visibly higher than the other side, you have vertical displacement. This is the most serious category and typically means the slab has actually broken, not just cracked. Call a structural engineer or a foundation specialist this week.

One San Antonio specific note: the city’s building code requires a soils report for new ADU construction precisely because of this clay movement, but older ADUs, especially pre-2015 accessory structures and garage conversions, were often slabbed without one. If your ADU was built before the current ADU ordinance and you’re seeing diagonal cracks, the fix may be more than polyurethane injection. The ADU Builders in San Antonio who understand local soil conditions will tell you the same thing: measure first, panic second.

Mini-Split Warning Signs San Antonio Homeowners Miss

Most San Antonio ADUs run on ductless mini-split systems because they’re efficient, quiet, and don’t require the ductwork a full HVAC system needs. But mini-splits fail differently than central air, and most homeowners don’t know what they’re hearing or seeing until the system is already damaged.

These are the four symptoms we tell every ADU owner to pay attention to:

1. A gurgling or bubbling sound from the indoor unit. That’s refrigerant moving through a line set with air in it, or a line set that’s undercharged. A properly charged mini-split is nearly silent. Gurgling means the system is losing refrigerant somewhere, and because mini-split line sets can be 50 feet long with multiple flare connections, the leak could be anywhere along that run. Refrigerant leaks don’t fix themselves.
2. Uneven cooling between two rooms on the same system. A single outdoor condenser can drive two or three indoor heads. If one room hits 72 degrees while another won’t get below 78, you may have a failing expansion valve on one head, a kinked line set, or a refrigerant charge that has dropped to the point where the system can’t feed all heads evenly. This is not a “turn it down and wait” problem.
3. Ice forming on the line set during summer operation. In San Antonio, where July highs sit in the mid-90s, an iced-over line set means the evaporator coil isn’t absorbing heat properly. The cause is usually a dirty coil, a failing blower motor, or low refrigerant. Running the system with ice on the lines forces the compressor to work against itself and shortens the life of the outdoor unit.
4. The outdoor unit runs constantly and the indoor fan never cycles off. A right-sized mini-split in a well-insulated ADU should cycle. If it’s running flat-out from noon to 8 p.m. every day, the system is either undersized for the square footage, the ADU is leaking conditioned air, or the inverter board is failing and the compressor won’t modulate down. All three are fixable, but none of them get better on their own.

Mini-split maintenance in San Antonio should include cleaning the washable filters monthly during cooling season and having the line set pressure-tested every two years. If you’re building new, ask your contractor about the specific mini-split brand and capacity they’re pairing with the ADU shell before you sign. A properly sized and charged mini-split on a prefab ADU from a brand like Dvele or Cover should hold temperature without breaking a sweat, even in August.

Exterior Envelope Red Flags in San Antonio Humidity

San Antonio doesn’t get Houston humidity, but it gets enough. Between May and September, dew points sit in the 70s, and any weakness in your ADU’s exterior envelope shows up during those months. The trick is knowing what the evidence is telling you, because paint failure and water intrusion look similar and get treated very differently.

Fiber cement siding, which includes James Hardie products used on many ADU builds in San Antonio, fails in predictable ways. Wood trim fails in different ways. Here’s what each symptom means:

• Paint bubbling at the bottom 12 inches of the wall: This is bulk water from splashing rain or irrigation spray. The bottom edge of fiber cement siding acts like a wick, and if the siding was installed too close to grade (less than 6 inches of clearance is a common ADU construction shortcut), every rain event soaks the cut edge. The fix is usually grading and clearance, not repainting.
• Paint bubbling or peeling at the top of the wall, under the eave line: This signals vapor drive, not a leak. Warm, humid San Antonio air hits the cool inside of the wall cavity, condenses, and the moisture pushes outward through the siding. If the ADU was built without a proper vapor barrier or with one installed on the wrong side of the wall for our climate zone, this pattern appears within the first three years.
• Efflorescence, the white chalky residue, on a stem wall or foundation: Efflorescence is minerals left behind when water evaporates from concrete. It looks cosmetic, but it means water is actively moving through your foundation wall. On a San Antonio clay lot, that water is often trapped against the stem wall by poor drainage, and it’s the same water that eventually causes the clay to swell unevenly and crack the slab. Kill the water source first.
• Wood trim that stays dark after rain or shows soft spots at joints: Wood trim on an ADU rots from the end grain first. If the trim around windows or at inside corners stays visibly damp for 48 hours after a rain, the joint flashing failed or was never installed. By the time you can push a screwdriver into the wood, the rot has likely spread behind the siding.
• Caulk separating from around windows within one year of installation: Caulk is the last line of defense, not the first. If it’s separating that fast, the window flange wasn’t flashed properly behind the siding, and water is getting past the outer layer. Re-caulking buys you six months. Flashing repair fixes it.

If your ADU is a factory-built model from Mighty Buildings or Boxabl, the envelope details were handled in a controlled factory environment, which eliminates a lot of these field-installation failures. Site-built ADUs or garage conversions are where we see the flashing, clearance, and vapor barrier shortcuts most often. Either way, a free envelope inspection is the fastest way to know whether you’re looking at a paint problem or a water problem.

Electrical Warning Signs When an ADU Shares a Panel

When an ADU is wired as a subpanel off the main house panel, which is how most San Antonio ADUs are set up, the electrical symptoms look different than they do on a standalone service. A tripping breaker on a shared panel is rarely “just the breaker.” Here’s what’s actually happening in the four scenarios we see most often on San Antonio ADUs.

The GFCI trips every morning, usually between 6 a.m. and 9 a.m. A ground-fault circuit interrupter trips when it detects current leaking to ground, and the morning pattern is the clue. Morning means dew, irrigation, or a temperature change. In most cases we’ve traced this to an exterior outlet or a mini-split disconnect that’s collecting moisture overnight, then drying out enough during the day to reset. The fix is usually a weatherproof cover or a wet-location-rated outlet, not replacing the GFCI.

Lights dim when the mini-split compressor kicks on. A brief, subtle dim is normal when a compressor starts, especially on a shared panel where the ADU is at the end of a long feeder run. But a pronounced dim, or lights that flicker while the system runs, means voltage drop on the feeder. The ADU may be pulling more than the subpanel feeder was sized for, or the neutral connection at the main panel has loosened. A loose neutral is a fire hazard and should be checked this week.

One breaker, usually a 15-amp bedroom circuit, trips on hot afternoons. This one is San Antonio specific. The circuit shares a wall cavity with the west-facing exterior wall, and the afternoon sun heats the wall cavity enough to increase wire resistance. If the circuit is already loaded near capacity, the added thermal load pushes it over. The real fix is redistributing the load or adding a dedicated circuit, because the underlying problem is that the ADU was wired near its design limit.

The main panel breaker feeding the ADU subpanel trips once a month or so. This is undersized service. The ADU is pulling more total amps than the subpanel feeder was designed for, and the breaker at the main panel is doing its job. The fix is a service upgrade or a load calculation that moves demand off the shared feeder. If you’re considering adding an EV charger, a heat pump water heater, or a second mini-split head, have an electrician recalculate the load first.

One thing we always mention: if your ADU was built as part of a Home Addition Contractor in San Antonio project, the electrical may have been permitted and inspected with the main house. If it was a garage conversion from the 1990s or earlier, the electrical may never have been pulled as a separate permit, and the subpanel could be undersized. That’s worth knowing before you add load.

Tenant Complaints That Are Actually Structural Warnings

Most tenant complaints sound like comfort issues. Many of them are actually early warning signs of moisture, foundation, or ventilation problems, and the translation is pretty consistent. Here’s the builder’s guide to decoding what a San Antonio ADU tenant is actually reporting.

• “It smells musty.” Translation: you have active moisture somewhere, and in San Antonio, the most common source in an ADU is not a leak. It’s the crawl space or the slab edge wicking ground moisture into the wall cavity. Musty means mold or mildew is growing, and that needs a source-of-water fix, not an air freshener. If the ADU sits on a vented crawl space, check the vapor barrier. If it’s on a slab, check whether the grade slopes toward the foundation.
• “My allergies are worse since I moved in.” Translation: the same thing, but earlier in the timeline. Before visible mold appears, relative humidity above 60 percent in the living space triggers dust mite and mold spore activity. Get a $20 hygrometer, leave it in the ADU for a week, and look at the highs. If it’s regularly above 60 percent inside, the envelope has a moisture problem even if nothing is visibly wet.
• “The door sticks sometimes but not other times.” Translation: the slab is moving, or the door frame was installed without seasonal adjustment room. If the sticking tracks with rain and dry spells, it’s slab movement on clay. If it correlates with the afternoon sun on the west wall, it’s thermal expansion of the door itself. Track it for 90 days with dated photos before you plane the door, because planing a door on a moving slab just means you’ll have a gap the next season.
• “It never gets below 78 in the afternoon.” Translation: the mini-split is undersized, the envelope is leaking air, or the tenant is running it wrong. Before you replace equipment, check the obvious: is the ADU insulated to San Antonio’s climate zone requirements, are the weatherstrips intact, are the windows single-pane? An undersized system in an over-leaky shell can’t keep up, and no amount of thermostat adjustment changes that.
• “There’s water pooling by the wall after it rains.” Translation: grading failure. Water is running toward the foundation instead of away from it, and on San Antonio clay, that water is doing two things at once: soaking into the clay and creating expansion pressure on the slab, and finding its way into any crack or cold joint in the foundation. Fix the grade now, because the foundation repair is the expensive version of the same problem.

If you rent out your ADU, document every one of these complaints with a walkthrough. The tenant is your first-line inspection system, and they’re in the unit 16 hours a day. What they notice first is often what a builder would notice in a driveway walkaround.

Sticking Doors and Windows: Seasonal or Structural?

A door that sticks in a San Antonio ADU is either the most overrated warning sign in home construction or one of the clearest, depending on when it happens. The timing tells you everything.

Sticks in July, fine in January: This is seasonal movement on clay soil. During a dry San Antonio summer, the clay under the slab shrinks, the slab settles slightly and unevenly, and door frames that were square in February go slightly out of square. The door binds at the top corner. When the rains return in the fall, the clay rehydrates, the slab lifts back up, and the door swings free. This is normal, but it’s also telling you that the slab is moving enough to notice, which means you should be tracking foundation cracks on the same schedule.

Sticks year-round, same door, same spot: This is either an installation problem (the frame was set out of square and shimmed) or the door itself has warped from humidity exposure. On a San Antonio ADU with a west-facing exterior door, the outside face bakes while the inside face stays conditioned, and a solid wood door will cup permanently. Hollow-core or composite doors resist this much better.

Window won’t open or close smoothly, but only at certain times of day: Vinyl window frames expand in direct afternoon sun. If a south or west-facing window binds from 3 p.m. to sunset and operates fine in the morning, that’s thermal expansion, not foundation movement. If it binds all the time and the corner of the frame shows a diagonal drywall crack nearby, that’s slab movement affecting the rough opening.

The practical routine: photograph the sticking door and the gaps around it every quarter, and note whether the sticking shifts with the seasons. One year of documentation gives you the answer without paying for a structural opinion you may not need. If you do end up needing one, most Room Addition Contractor in San Antonio firms can refer you to a local structural engineer who works on ADU-scale projects regularly.

Common Mistakes to Avoid

• Caulking over a crack and calling it fixed. Caulk seals the surface, but it doesn’t address the movement that created the crack. On a moving slab, the caulk will fail within a season, and you’ll have masked a foundation symptom for six months. Measure and track the crack first.
• Assuming a tripping GFCI is a bad outlet. A GFCI that trips repeatedly is doing its job by detecting a ground fault. Replacing it without finding the fault just moves the problem to a new device. The moisture source is the issue.
• Running a gurgling mini-split because “it still cools.” A mini-split losing refrigerant will continue to cool while the leak is small, but it’s running the compressor hot and burning through its service life. By the time it stops cooling, the repair is often a compressor replacement instead of a leak fix and recharge.
• Ignoring a door that sticks to “fix later.” In San Antonio, a seasonally sticking door is a free foundation monitoring system. If you plane the door, you lose the signal. Track it instead, and let the pattern tell you whether the slab is moving.
• Venting a bathroom fan into the attic. This is common in older San Antonio ADUs, and it pumps warm, humid air into a space that was never designed for it. The result is mold on the underside of the roof deck and moisture migrating down through the ceiling. The fan needs to terminate outdoors through a proper vent cap.
• Waiting for visible damage before addressing tenant complaints. By the time a water stain appears on the ceiling or a crack opens to a quarter inch, the underlying problem has been running for months. Tenant comments about smells, sticking doors, or temperature swings are the early warning system. Treat them that way.

When to Call a Professional

Some symptoms are watch-and-track, some are call-us-this-week, and a few are call-us-today. Call this week if you have: a diagonal slab crack wider than 1/8 inch that has grown in the last 90 days, a mini-split that gurgles continuously or shows ice on the line set in summer, a GFCI that trips every morning without an obvious exterior outlet at fault, or paint bubbling at the bottom of a wall after rain. Call today if you see: vertical displacement across a slab crack, water pooling against the foundation during or after rain, a burning smell from any outlet or the subpanel, or a tenant reporting damp spots that persist more than 48 hours. Benson ADU San Antonio offers free estimates throughout San Antonio, and we’ll walk the ADU with you, classify what we find, and put the next steps in writing. Call (430) 258-2928 and a live person will pick up.

For a broader look at how all of this fits together, start with the Benson ADU San Antonio home page, then explore what goes into building an ADU the right way on our ADU Builders in San Antonio guide. If you’re weighing an addition instead of a standalone unit, our Home Addition Contractor in San Antonio and Room Addition Contractor in San Antonio pages cover the same ground for attached projects.

The Bottom Line

Every ADU in San Antonio is talking to you, and most of the time it’s using a language you can learn in one afternoon. The slab tells you about soil movement through crack direction and width. The mini-split tells you about refrigerant and load through sound and ice. The paint tells you about water through where it bubbles. The breaker tells you about wiring through when it trips. And the tenant tells you about all of it, usually first. The difference between a $300 fix and a $15,000 repair is whether you catch the symptom in its first season, before the pattern compounds. That’s the whole ballgame. Watch it, measure it, photograph it, and when the threshold crosses from “track this” to “call now,” pick up the phone.

Written by Troy Eastman, Owner at Benson ADU San Antonio, serving San Antonio since 2006.