ADU Builders Troubleshooting: Common Problems and How to Fix Them

ADU Builders Troubleshooting: Common Problems and How to Fix Them

A soft spot in your ADU floor three months after move-in is either normal subfloor settling or the beginning of a moisture problem that costs $8,000 to fix. The difference is visible if you know what to look for. Most ADU construction problems don’t announce themselves as emergencies. They start as small things a homeowner notices and isn’t sure whether to flag: a door that’s slightly off-plumb, a ventilation fan that runs constantly, a floor that feels soft in one corner. This guide teaches you to read those signals correctly so you know when to call your builder back and when you’re just experiencing new-house settling. We’ve walked through hundreds of ADU walkthroughs and warranty callbacks across Los Angeles, from Sunland-Tujunga to Westchester, and the pattern is consistent: homeowners who document early and communicate clearly get problems resolved faster, cheaper, and with less friction. For more on what to watch for before you even start, see our ADU Builders Warning Signs Every Homeowner Should Know.

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Quick Answer

Most ADU construction problems fall into four categories: moisture intrusion, HVAC performance issues, electrical nuisances, and finish defects. Roughly two-thirds are cosmetic or minor and can wait for a scheduled warranty visit; the remaining third involve water, structure, or electrical safety and need attention within days, not weeks. In California, your legal standing comes from the implied warranty of habitability plus any written workmanship guarantee your builder provided, and the Calderon process gives you a specific legal path for defect claims before you ever file a lawsuit.

Table of Contents

• Moisture and Waterproofing Problems
• HVAC Performance Issues in Small Spaces
• Electrical Nuisances and When They Signal Real Trouble
• Finish Defects: Cosmetic vs. Structural
• Garage Conversion-Specific Problems
• Your Legal Standing: California’s Implied Warranty and the Calderon Process
• How to Document Defects and Write a Notice That Protects Your Rights
• What a Written Workmanship Guarantee Should Cover

Moisture and Waterproofing Problems

Moisture is the most expensive problem category in ADU construction because damage compounds silently. In Los Angeles, the risk profile is specific: we get concentrated winter rains against stucco and siding, marine-layer humidity near the coast, and long dry stretches that let small leaks evaporate before anyone notices them. By the time a homeowner sees a stain on drywall, the framing behind it may have been wet for months.

The most common moisture problems we see in Los Angeles ADUs, in rough order of frequency:

• Improperly flashed roof-to-wall intersections where an ADU ties into an existing garage or house. These show up as water stains near the top of interior walls after the first heavy rain.
• Failed or missing weep screed on stucco walls, which traps water behind the stucco and rots the sheathing from the inside out. You’ll see paint bubbling or efflorescence, a white chalky residue, near the base of exterior walls.
• Slab edge leaks caused by grade sloping toward the foundation instead of away from it. The symptom is a damp smell in the ADU after rain, with no visible interior water.
• Shower pan failures in ADU bathrooms that were waterproofed with a single membrane layer rather than a full waterproofing system. The tell is loose or hollow-sounding tile near the shower curb.
• Condensation on uninsulated cold water lines running through unvented floor cavities, which drips onto subflooring and creates slow rot.

How to Read the Signal

A soft floor near an exterior wall, a bathroom, or a kitchen is almost always moisture-related until proven otherwise. A soft floor in the center of a room with no plumbing nearby is more likely subfloor deflection, which is a different conversation with your builder. Here is the quick diagnostic we recommend to Los Angeles homeowners:

1. Mark the soft spot with painter’s tape.
2. Check whether it is within three feet of any plumbing fixture, exterior wall, or door.
3. Check whether the softness changes after rain versus after a dry week.
4. Take a photo of the area from three angles, including one from underneath if there is a crawl space.
5. Call your builder. Do not wait. Moisture problems do not self-correct.

If you are inside the guarantee period on a Kingswell-family project, a documented moisture concern triggers a site visit and a written finding. That is the standard homeowners should expect from any builder, and it’s worth asking directly: “What is your written process for water-related callbacks?”

HVAC Performance Issues in Small Spaces

ADUs are small. That is the entire challenge. A 400-square-foot accessory dwelling unit has almost no thermal mass, so HVAC problems that would be minor in a 2,000-square-foot house become impossible to ignore. The two most common complaints we hear in Los Angeles are temperature swings between rooms and a mini-split system that never seems to stop running. For a full picture of what to budget across every category, see our ADU Cost Breakdown: The Los Angeles Homeowner’s Reference for 2026.

The temperature swing problem usually comes down to one of three causes: the system was sized using rough square-footage rules instead of a proper Manual J load calculation; the supply air is not reaching the far corner of the unit because the duct run is undersized or has been crushed; or the ADU has a large west-facing window in Los Angeles where afternoon sun loads can exceed 1,000 watts per square meter in July and August.

The “never stops running” complaint is more nuanced. A properly sized variable-speed mini-split is designed to run nearly continuously at low output, maintaining a steady temperature instead of cycling on and off. That is not a defect. The defect version is when the unit runs constantly at full output and the room still does not reach the set point, which is almost always an undersized system, low refrigerant, or a failed inverter board.

What Homeowners Should Test

• Measure the temperature at the supply register and at the return grille. A healthy split is 15 to 20 degrees between them.
• Close the ADU door and check whether the system can hold 72 degrees on a 90-degree Los Angeles afternoon within two hours.
• Listen for the outdoor unit short-cycling: turning on and off every few minutes rather than running steady. That points to an oversized system or a sensor problem.
• Check whether the outdoor unit is placed against a south or west wall in full afternoon sun, which reduces efficiency by 10 to 15 percent and shortens compressor life.

Unlike moisture problems, HVAC performance issues rarely need emergency response unless the system has failed entirely during a heat wave. What they need is documentation: thermostat photos, temperature readings, and a log of when the system runs and what it does. That paper trail is what separates a fast manufacturer warranty claim from a slow one.

Electrical Nuisances and When They Signal Real Trouble

Most electrical complaints in new ADUs are annoyances, not hazards: a breaker that trips when the microwave and the toaster run at the same time, an outlet that lost power, a dimmer that buzzes. But in an accessory dwelling unit, a few specific electrical symptoms deserve immediate attention because they can indicate improper work at the subpanel or at the connection to the main house.

The most serious one we see in Los Angeles is a neutral wire problem. The symptoms are strange: lights brighten and dim on their own, one half of an outlet works while the other doesn’t, or a TV displays rolling interference while an appliance cycles. A loose neutral at the subpanel can cause voltage swings between 90 and 170 volts, which slowly destroys electronics and is a genuine fire hazard. If you see lights flickering rhythmically when a large appliance kicks on, call an electrician that week, not that month.

Tripping AFCI or GFCI breakers are common and usually nuisance-level, but they should still be logged. Arc-fault breakers are required in nearly all living spaces in California ADUs, and older appliances with brushed motors, like some vacuums and treadmills, can trip them. If a specific breaker trips repeatedly with nothing plugged in, that is a wiring fault, not a nuisance.

A Quick Diagnostic Sequence

1. Identify which breaker controls the affected circuit.
2. Unplug everything on that circuit and reset the breaker.
3. If it holds, plug items back in one at a time to identify the offender.
4. If it trips with nothing plugged in, leave it off and call a licensed electrician.
5. Photograph the breaker label and the subpanel interior so you can describe the setup accurately.

One Los Angeles-specific note: many older homes in neighborhoods like Leimert Park, Highland Park, and Reseda have 100-amp main services. Adding an ADU with a 50-amp subpanel load on a 100-amp service is legal with load management, but it leaves almost no headroom for future needs. If your main service is old and you’re planning an ADU, ask the builder to evaluate whether a service upgrade to 200 amps should be part of the scope from day one, rather than a surprise later.

Finish Defects: Cosmetic vs. Structural

Every new ADU will have some finish defects. That is not a builder excuse; it is the reality of assembling thousands of components on a construction site. What separates a normal punch list from a real problem is whether the defect is static or moving.

Static defects are what they look like: a paint drip, a nail pop, a grout line that dried a shade off, a cabinet door that needs one hinge adjustment. They appear once and never change. Cosmetic. They should be documented and fixed at the scheduled warranty walkthrough.

Moving defects change over time: a crack that grows, a floor gap that widens, a door that becomes progressively harder to close. That is the signal to watch. In Los Angeles, minor hairline cracks at drywall joints are common in the first year as the building dries and settles, especially after the wet winter-to-dry summer cycle. A hairline crack that stays hairline is cosmetic. A crack that opens to a quarter inch over six months, or a door that goes from rubbing slightly to not closing at all, means something under the surface is shifting: foundation movement, improper framing, or a missing structural element.

Here is the rule we give homeowners: measure and date everything. When you find a crack, put a piece of painter’s tape across it with the date written on it, or photograph it with a ruler held against the surface. Recheck in 30 days. A builder should welcome that habit because it replaces subjective arguments with data.

Garage Conversion-Specific Problems

Garage conversions are the largest share of ADU work in Los Angeles, and they carry a distinct problem set that homeowners rarely anticipate. A garage was never built to be lived in. The slab, the wall framing, the door opening, and the original insulation, if any, were all designed for storing cars, not for human comfort. The problems cluster in predictable places:

• Insufficient insulation at the former garage door wall. Builders replace the door with a framed wall, but if that wall is insulated to less than the required R-13 for Los Angeles climate zone, or if the wall-to-foundation junction is left unsealed, that corner of the ADU will be cold in winter and hot in summer, and the homeowner will feel it immediately.
• Condensation on the slab. Existing garage slabs typically have no vapor barrier beneath them because they were poured for vehicle storage. When you install flooring and start living on that slab, moisture migrates up through the concrete and condenses under vinyl plank or laminate, causing warping, musty smell, and eventually mold. The fix is flooring underlayment with a built-in vapor barrier, or sealing the slab before flooring goes down. Cheap builders skip this step; good ones don’t.
• Ceiling height clearance issues discovered at final inspection. California requires a 7-foot minimum ceiling height in habitable rooms. Some older Los Angeles garages have ceiling joists at 6’8″ or lower, which means the conversion needs structural modification, usually raising the joists or reframing the roofline, and that is a five-figure scope change discovered late in the project.
• Inadequate floor elevation at the garage door threshold. Garages are built with a slight slope toward the door for drainage. When that opening is walled off, water can still find the old low point and seep under the new wall, showing up as damp baseboards on the first rainy day.

If you are planning a garage conversion in Los Angeles, ask the builder during the estimate phase, before you sign anything: “What is the existing slab condition, and is there a vapor barrier?” “What is the existing ceiling height, and does it meet code?” “What insulation will go into the former garage door wall?” A builder who cannot answer those questions specifically has not done the site analysis. The Garage Conversion Contractor in Los Angeles page on our site walks through what to look for in that evaluation.

Your Legal Standing: California’s Implied Warranty and the Calderon Process

California law gives every homeowner a baseline of protection regardless of what the contract says. The implied warranty of habitability means a builder is responsible for defects that make a home uninhabitable or unsafe, even if the defect was not specifically listed in the contract. That includes significant water intrusion, structural problems, electrical hazards, and plumbing failures that affect livability. It does not extend to cosmetic issues like paint shade differences or nail pops.

For new construction defects, including ADUs, California’s SB 800, known as the Calderon process or the “Right to Repair” law, establishes a specific procedure before any litigation. Homeowners must notify the builder in writing of the claimed defects, and the builder has a defined window, typically 14 days to acknowledge and then a set inspection and repair timeline, to respond. The law sets standards for different building components: for example, a roof leak window of 2 to 4 years depending on the cause, and a 10-year outer limit for latent defects. What the Calderon process gives you is a framework that keeps the conversation structured and documented, which protects you whether you end up working it out with the builder or going further.

The practical takeaway for a Los Angeles homeowner: you have stronger legal footing than you probably realize, but only if you follow the process. Unwritten conversations, voice mails, and handshake promises do not trigger your protections. Written notice does.

How to Document Defects and Write a Notice That Protects Your Rights

Documentation is the difference between a defect that gets fixed and a defect that turns into a dispute. Here is what to do the moment you notice something wrong, in order:

1. Photograph everything. Wide shot for context, close shot for detail, and one with a ruler or tape measure showing the size of the defect. Date-stamp the photos or save them in a folder with the date in the name.
2. Write down what you observed, when, and under what conditions. “Water stain above the west bedroom window, noticed March 3 after heavy rain on March 1-2” is a hundred times more useful than “water stain.”
3. Send written notice to the builder. Email is fine and creates a timestamp. Describe the defect, attach photos, and state plainly what you are asking for: inspection, repair, or both.
4. Reference the guarantee. If your project came with a written workmanship guarantee, refer to it by section or promise. That shifts the conversation from a favor to a contractual obligation.
5. Give a reasonable response window, then follow up in writing. For non-emergency defects, 14 days is a fair window to expect an acknowledgment. For water intrusion or electrical safety issues, that shrinks to 48-72 hours.

A reasonable builder response timeline looks like: acknowledge receipt within 3 business days, schedule an inspection within 2 weeks for non-urgent items, and provide a written finding with either a repair plan or an explanation of why the issue is not covered, within 30 days of the inspection. If a builder goes silent for weeks, that is itself a data point about whether they intend to honor their work. This is also where the value of a written guarantee becomes obvious: it removes the ambiguity about what “reasonable” means.

What a Written Workmanship Guarantee Should Cover

A written workmanship guarantee is the single most useful document a homeowner can have when a problem surfaces, and it is worth reading before you sign, not after. Here is what a legitimate one covers and what exclusions are fair versus what exclusions are loopholes.

What should be covered:

• Labor and materials to correct defects in workmanship for a stated period, typically one year minimum for workmanship in California.
• Waterproofing and moisture barrier integrity at all penetrations: windows, doors, roof intersections, plumbing chases.
• Structure: foundation movement within accepted tolerance, framing, shear wall connections, and roof structure.
• Electrical and plumbing system function as installed, including fixtures, outlets, and connection points.
• HVAC system operation as installed, with manufacturer warranties handled separately for the equipment itself.
• Finish work: paint, tile, flooring, cabinetry, and trim installed to a stated standard.

Legitimate exclusions include normal wear and tear, damage caused by homeowner misuse or alteration, acts of God like earthquakes and floods, and manufacturer defects in appliances or equipment that carry their own warranty. A guarantee that excludes room-temperature fluctuations of a few degrees or standard settling cracks under a stated width, say 1/16 inch, is being honest about real-world construction.

Loopholes to watch for: a guarantee that requires the homeowner to notify “within 7 days of noticing” but gives the builder unlimited time to respond; a guarantee that covers “labor only” but not the cost of accessing the defect, such as opening and patching drywall; a guarantee that excludes all exterior work, all roof work, or all plumbing; and a guarantee that requires binding arbitration in a distant venue with the homeowner paying all fees. Those are designed to make callbacks expensive and slow.

The 365-Day Done Right Promise that ADU Builders services like Lodgewick provides is written on every project before work begins, and it includes a free return visit if the work fails within the guarantee period. The Kingswell Way is the operating standard behind it: say the price before the work, show up in the window given, leave the place cleaner than found, put the guarantee in writing, and come back if it fails. If your current builder’s guarantee does not match that plain-language standard, you have your answer about how hard it will be to get a callback honored. We work with leading ADU manufacturers like Cover, Dvele, and Boxabl, and we coordinate directly with their factory warranty teams when a component issue arises, so you are not stuck in the middle between builder and manufacturer.

Common Mistakes to Avoid

• Waiting to document because you do not want to be a bother. Builders would rather see a hairline crack at month 2 than a structural failure at month 14. Early documentation protects both sides and is the single biggest factor in smooth warranty resolutions.
• Accepting a verbal promise as a fix. “We’ll take care of it” without a written follow-up gives you nothing if the crew changes, the company restructures, or the promise is forgotten. Ask for every commitment in writing, even a one-line email.
• Attempting DIY fixes before the builder inspects. Once you patch, seal, or modify a defect, the builder can legitimately claim the original condition cannot be assessed and deny the warranty claim. Leave everything as-is until after the walkthrough.
• Confusing manufacturer warranty with workmanship guarantee. A faulty mini-split is the manufacturer’s responsibility; the installation of that mini-split is the builder’s. When a system fails, both parties may need to be involved, and understanding which is which speeds everything up.
• Skipping the pre-drywall inspection. If you are building an ADU in Los Angeles, insist on walking the project after rough framing, electrical, and plumbing are in but before drywall goes up. Take photos of every wall and ceiling. That photo set becomes your most valuable reference for the life of the building.
• Not asking about the vapor barrier on a garage conversion. In Los Angeles, where garage conversions represent a large share of ADU permits, this one question can save you from a ripped-out floor and a mold remediation bill later.

When to Call a Professional

Call a professional the same week for any of these: water stains on ceilings or walls, any visible mold growth, electrical symptoms like flickering lights or warm outlet faces, any crack wider than a quarter inch, doors or windows that will not close, or any gas smell anywhere in or near the ADU. Call within 48 hours for structural movement you can see changing week to week. And call immediately if a breaker keeps tripping with nothing plugged in or any outlet feels hot to the touch. Everything else, from nail pops to grout shade to a noisy fan, can wait for a scheduled visit. Lodgewick ADU Builders offers free estimates in Los Angeles, and a live person answers the phone at (254) 602-5618 any time you need a second opinion on whether something is a real problem.

Frequently Asked Questions

The Bottom Line

ADU construction problems are rarely mysteries; they are signals that respond to consistent, documented attention. Water at the wrong place, a crack that moves, a breaker that trips with nothing plugged in, a room that will not hold temperature: each one tells you something specific about the building, and each one has a best response time measured in days, weeks, or months. Your strongest protection is a written guarantee with clear callback terms, a photo log that starts before drywall, and the habit of putting every concern in writing. In Los Angeles, where ADU construction ranges from studio conversions in West Adams to two-story additions in Sherman Oaks, the homeowners who fare best are the ones who treat warranty communication as part of the build process, not an afterthought. The ADU Builders in Los Angeles page has more on how we approach that process, and our Lodgewick ADU Builders home page covers the full scope of what we build. If you are considering an addition, the Home Addition Contractor in Los Angeles page covers the adjacent territory.

Written by Russ Delmar, Owner at Lodgewick ADU Builders, serving Los Angeles since 2010. For more guides & resources, visit our blog.