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

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

A diagonal crack above a door rough opening is not a settling quirk. It is a load-path signal, and in an ADU built without proper header sizing, it appears within 18 months of occupancy. Most ADU problems announce themselves twice before they become expensive. The first announcement is visual, subtle, and easy to dismiss as cosmetic. The second announcement is a repair bill. This guide gives you the list no one handed you at move-in: what to look for, where to look, and how to tell the difference between a drywall crack and a structural warning in your San Jose ADU.

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Quick Answer

The most common ADU warning signs in San Jose fall into five categories: diagonal cracks at door and window corners (framing or foundation movement), moisture staining on lower drywall (slab or plumbing issues), a musty smell that comes and goes (ventilation or hidden leak), breakers that trip when two appliances run at once (undersized electrical panel), and doors that stick only in summer or winter (seasonal movement). A diagonal crack wider than 1/8 inch, or any crack that grows over 30 days, warrants a professional inspection. Most of these signs are fixable before they become structural if you document them early.

Table of Contents

• Foundation Warning Signs
• Framing Warning Signs
• Roof and Ceiling Warning Signs
• Plumbing Warning Signs
• Electrical Warning Signs
• Garage Conversion ADU Warning Signs
• What Unpermitted Work Looks Like After the Fact
• The Warning Sign Log: A Simple Documentation Template
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions
• The Bottom Line

Foundation Warning Signs

San Jose sits on two types of soil that behave differently under an ADU: the expansive clay common in the eastern foothills around Evergreen and Silver Creek, and the sandy loam closer to the Guadalupe River corridor. Clay expands when wet and shrinks when dry, which means foundation movement in San Jose often follows the rain season, not the calendar year. Your first look at an ADU foundation should happen after the first heavy rain of winter.

Here are the specific signs to look for:

• Stair-step cracks in exposed stem wall or skirting. These follow the mortar joints in a block foundation or the pour lines in concrete. A hairline stair-step is common in the first year. A stair-step wider than 1/8 inch, or one that reopens after patching, means the soil under one corner of the ADU is moving at a different rate than the rest.
• Gap between the ground and the bottom of the siding. When clay soil swells under part of the slab, that corner lifts and pulls the siding away from grade. You will see a tapering gap, wider at one end. If the gap opens and closes with the seasons, you are dealing with expansive soil movement, not a sinking slab.
• Windows or doors that stick only in winter or only in summer. This is seasonal movement. A door that sticks year-round and gets worse points to a different problem: a pier that is failing or a slab that has cracked through.
• Soil pulling away from the foundation edge. In hot, dry San Jose summers, clay soil contracts and can leave a visible gap between the soil and the concrete. This is less alarming than swelling soil, but it tells you the ground is active. A well-built ADU foundation in San Jose should have been designed for this, which is why we look at the site drainage plan as closely as the foundation itself.

One more San Jose-specific note: if your ADU sits within a mile of Coyote Creek or the Guadalupe River, ask whether the original builder installed a capillary break under the slab. Many older garage conversions in the Willow Glen and Naglee Park areas were poured before this practice was standard, and the slab now wicks groundwater up through the concrete. The symptom is not a crack. It is a floor that never quite feels dry.

Framing Warning Signs

Framing problems announce themselves at openings: doors, windows, and any place a wall changes height. A load-bearing wall transferring roof weight to the floor below will show stress at the weakest point, which is the header above an opening. If the header is undersized, the drywall above the opening will develop a diagonal crack starting at the upper corner and moving toward the ceiling. That is the load-path signal we mentioned in the opening sentence. It is not settling, and waiting a year to see if it grows is not a strategy.

What to look for:

• Diagonal cracks starting at door or window corners. These are the classic header-deflection cracks. They typically angle up and inward, and they will be on both sides of the opening if the header is the problem. Measure the width with a penny: a penny is about 1/16 inch thick. If the crack is wider than two pennies, call someone.
• Horizontal cracks along a wall that runs parallel to the roofline. This suggests racking, which means lateral movement, which in San Jose means either seismic activity or a wall that was not sheathed properly. San Jose is in a high seismic zone, and an ADU built cheaply may have OSB sheathing only on exterior walls, leaving interior shear walls too weak to resist lateral force.
• Nail pops in a line. A single nail pop is cosmetic. A line of nail pops running down a wall or across a ceiling means the framing member behind the drywall has shifted enough to push the fasteners out. The line happens because the member moved as a unit.
• Gap between the wall and the ceiling. A consistent, pencil-width gap that runs the length of a room is often the joint compound shrinking in the first year. A gap that is wider on one end than the other is support movement, and it belongs on your log.

If anyone tells you these cracks are normal in a new ADU, ask them to write that sentence down and sign it. In our experience in San Jose, most builders will not. The building code has specific deflection limits for headers, and a crack that appears within 18 months of occupancy usually means those limits were missed. For perspective on how a properly framed addition or ADU should behave, the standards we follow at Fielding ADU San Jose home are the same ones for larger room addition construction in San Jose: same header tables, same shear requirements, same inspection schedule.

Roof and Ceiling Warning Signs

Roof problems in San Jose ADUs are less about leaks than about two local factors: the intense summer sun and the occasional Pineapple Express storm that dumps inches of rain in a day. Heat degrades roofing materials faster here than in coastal climates, and the cold-water expansion after a 100-degree week can turn a slow seep into a visible stain.

Look for these signs:

1. Tenting or billowing at the roof edges. If the roofing membrane lifts at the edges when the wind blows, the edge flashing was never properly secured. On the next rain event, water enters at the eave and follows the rafter tails to the top plate. The stain appears on the ceiling near the wall, not in the middle of the room.
2. Ceiling stains that grow after rain but dry between storms. This is a slow leak at a penetration: vent pipe, skylight, or the junction where the ADU roof ties into the main house. The stain will have darker rings, like a tide line, because each storm deposits a little more mineral sediment.
3. Sagging between ceiling joists. In a flat-roof ADU, the ceiling is often the underside of the roof assembly. If the drywall sags between joists, water has been sitting on top of that drywall for weeks. This is a tear-out situation, not a patch situation, and you should not wait through another San Jose rainy season to address it.
4. Soffit vents painted over or blocked. A sealed soffit vent in a small ADU with a kitchen or bathroom means the moisture from cooking and showering has nowhere to go. You will see condensation on windows, a persistent musty smell, and eventually mold on the ceiling near the bathroom. This is the most common roof-adjacent problem we see in smaller San Jose ADUs, especially those built tight for energy efficiency.

One distinction that matters: a roof leak is an active problem, and a blocked vent is a slow problem. Both produce moisture on the ceiling. The difference is that a blocked vent produces moisture that moves around with your daily habits, while a roof leak produces moisture that follows the weather. If you live in San Jose and the ceiling stain appears in July after weeks with no rain, it is not the roof.

Plumbing Warning Signs

Plumbing problems in an ADU often show up as small pressure changes before they show up as water damage. A shower that runs cold the moment someone flushes a toilet in the main house, a kitchen sink that gurgles when the washing machine drains, or a water heater that can only fill two hot showers before running cold, all point to undersized supply lines or venting problems. These are the first announcements. The second announcement is a collapsed drain line or a slab leak.

The signs to document:

• Gurgling drains. When you hear a gurgle from a sink or shower drain after a flush or a washer cycle, the drain line is not venting properly. This is one of the most common findings in San Jose ADUs built by crews that treated the ADU plumbing like a second bathroom instead of a separate dwelling unit with its own wet-venting needs.
• Hot water runs out fast. A 30-gallon tank cannot support an ADU with a washing machine and two occupants taking showers. If the original build was permitted, the plan required a specific tank size or tankless setup based on fixture count. If the tank is small and the permit was never pulled, you are now discovering why the seller priced the ADU as a “bonus space.”
• Water pressure drops when two fixtures run. In San Jose, older neighborhoods like Rose Garden and Alum Rock still have 1/2-inch copper supply lines on some properties. A well-built ADU should have its own 3/4-inch or larger supply branch from the meter. If the ADU shares a 1/2-inch line with the main house, the pressure drop is permanent until the line is replaced.
• Damp drywall at the base of the shower or tub surround. This is often a failed shower pan membrane or a caulk joint that has separated. The water travels down behind the surround and puddles at the floor plate. If the drywall is soft to the touch, the framing behind it is already wet, and in a San Jose wood-frame ADU, that means wood rot and possibly mold in the wall cavity.

One repair note: a drain line that is backing up in an ADU is not always a clog. In a garage conversion with a raised floor, the builder may have run the drain line with too little slope to reach the house sewer. The symptom is exactly the same: slow drains, gurgling, occasional backup. If you snake the line and it clogs again in three weeks, you are looking at a slope problem, not a clog problem. This is the kind of issue a San Jose garage conversion contractor sees repeatedly in older conversions.

Electrical Warning Signs

Electrical problems in ADUs fall into two categories: undersized systems and unpermitted work. Both are common in San Jose, where many ADUs were added onto properties with older 100-amp main panels. The ADU’s electrical load was never recalculated for the additional square footage, and now the entire property is running close to capacity.

Here is what you are looking for:

• Breakers trip when two appliances run at once. An ADU with a kitchenette should handle a microwave and a coffee maker at the same time. If the breaker trips, the circuit is undersized or overloaded. If multiple circuits trip, the subpanel feeding the ADU is undersized.
• Lights dim when the air conditioner or space heater cycles on. This is a symptom of voltage drop on an undersized feeder line between the main panel and the ADU subpanel. It is not harmless. Consistent voltage drop shortens the life of every appliance in the ADU, especially computers, refrigerators, and HVAC control boards.
• Warm outlets or switch plates. A warm outlet is not normal. It means resistance somewhere in the connection, which means heat, which means eventual failure or fire. Turn off the circuit and have it looked at. Do not run a space heater on a warm outlet. This is the one condition on this list that deserves a same-week response.
• The meter does not match the panel. We cover this in the unpermitted work section below, but in short: if the main panel label says 200 amps and the meter socket says 100 amps, someone upgraded the panel without upgrading the service, and the utility never approved anything.

For San Jose homeowners in older neighborhoods like Willow Glen or Naglee Park, the original house service may be 100 amps. Adding a permitted ADU with a kitchen, laundry, and mini-split HVAC usually requires a service upgrade to 200 amps. If your ADU was built without that upgrade and the panel is still 100 amps, the system is running on borrowed capacity. It works until it does not.

Garage Conversion ADU Warning Signs

Garage conversions are the single largest category of ADUs in San Jose, and they carry their own warning-sign vocabulary. A garage was never built to house people. It was built to shelter a car. The concrete slab is thinner than a house slab, the walls are not insulated, the header over the garage door is sized for wind load and not much else, and the electrical panel, if there is one, was sized for a few outlets and a garage door opener. When you convert that structure into living space, every one of those original limitations becomes a potential problem.

Three what-to-look-for lists, one for each system:

Header Deflection Signs

• The original garage door opening now has a window or French doors in it. Look at the drywall above that opening. A hairline crack running horizontally above the opening, or diagonal cracks at the top corners of the window, means the header over the old garage door is deflecting under roof load. The header in a garage was sized for a garage door, not for a wall with windows and an interior finish. If the conversion left the original header in place and simply framed below it, the header may now be carrying more weight than it was designed for.
• A window in the old garage door opening that is difficult to open or close. The frame is being pinched by the header deflecting above it. This is the same mechanism as a sticking door, but it shows up faster in windows because window frames are less forgiving than door frames.

Slab Moisture Signs

• Garage slabs in San Jose were often poured on grade with no vapor barrier beneath them. Before the conversion, the concrete could breathe into an open garage. Now the slab is covered with flooring, and that trapped moisture comes up through the concrete and gets sealed under vinyl plank or laminate. The signs: warping at the seams of plank flooring, a musty smell that you notice only when you sit or lie on the floor, and efflorescence, a white powdery residue, at the base of the drywall.
• Peeling paint or bubbling laminate along the bottom edge of the converted garage walls. This is moisture wicking up from the slab into the wall assembly. If you see this in a San Jose garage conversion, check the exterior grading. Water may also be collecting against the foundation from a downspout that discharges too close to the wall.

Panel Ampacity Signs

• The original garage electrical panel, if it existed, was maybe 60 amps or 100 amps and fed from the main house. Now that panel is supposed to run a kitchen, a bathroom, a mini-split, and a laundry setup. If the panel label is scratched out or missing, and the breakers are a mix of brands, the panel was cobbled together over time. Look at the main breaker in the ADU subpanel. If it says 60, the ADU is running on less power than a standard apartment.
• Flickering lights when the microwave runs. Not dimming, flickering. That is an overloaded circuit, not an undersized feeder. Either way, the panel is not matched to the load the ADU is actually carrying.

The good news is that a garage conversion in San Jose can be brought up to standard. The work involves proper headers, insulation, vapor barriers, and often a subpanel upgrade. The bad news is that correcting it means opening walls, which makes early detection worth real money. When we price a garage conversion in San Jose, the written number includes the structural corrections, not just the finishes. That distinction is why the quote from one builder can be twice another’s: one is quoting the work, and the other is quoting the surface.

What Unpermitted Work Looks Like After the Fact

If the previous owner built the ADU without permits, or paid a crew that routinely skips inspection, the evidence is still on the site. You do not need a forensic engineer to spot it, and you do not need to open walls. You need to know where city inspection requirements leave traces in the physical work.

Here is what to look for:

1. Missing cleanout access. San Jose plumbing code requires a cleanout where the ADU drain line connects to the house sewer. If you cannot find a cleanout cap anywhere on the drain path, the drain was never inspected. The cap is not cosmetic; it is how a plumber clears blockages without cutting pipe. No cleanout means no permit on the sewer tie-in.
2. Drywall gaps at the electrical panel. An inspected electrical rough-in requires the panel to be flush with the finished wall. If there is a visible gap around the panel, or the panel cover is held on with drywall screws, the panel was installed after finish, which is a telltale of unpermitted electrical work.
3. The meter does not match the panel. If your main service panel says 200 amps but the meter socket on the outside of the house still says 100 or 125 amps, the panel was upgraded without a service change, and the utility never signed off. The meter is the utility’s stamp of approval. If the numbers do not match, the approval never happened.
4. No tamper proof sticker on the water heater or mini-split? In San Jose, installed mechanical equipment gets an inspection sticker from the city. Older equipment may have lost the sticker, but a unit installed within the last five years with no sticker and no permit number on the furnace or heat pump suggests the mechanical work was never inspected.
5. Exposed Romex running along the surface of a wall. Interior electrical wiring must be protected inside the wall or run in conduit. Surface-run Romex stapled to the drywall is a code violation and a clear signal that whoever did the work did not plan to have it inspected.

Unpermitted work does not automatically mean the ADU is unsafe, but it does mean the work was never independently verified, and that is the point of a permit. In San Jose, an unpermitted ADU also complicates any future sale, insurance claim, or refinance. The lender’s appraiser will note it, and sometimes the deal dies at the closing table over a missing building record. Building to The Garrison Code means the permit is part of the price, and the inspection is scheduled, not skipped. That is the standard we follow at Fielding ADU San Jose, and it is the same standard that has been in place since 2006 under the Garrison Partners flag.

The Warning Sign Log: A Simple Documentation Template

The most productive conversation you can have with a contractor starts with facts, not impressions. A crack that has been measured is a data point. A crack that is “bigger than I remember” is an impression. Here is a simple log format that takes two minutes per entry and gives any contractor, engineer, or inspector exactly what they need. Keep it on your phone or on a legal pad taped inside a kitchen cabinet.

The entry format:

1. Date and time. Write it down. Memory does not hold dates.
2. Location. Be specific: “above the bedroom window on the west wall,” not “in the bedroom.”
3. What you see. Describe it plainly: “diagonal crack, right side of window, moving up-right toward the ceiling.”
4. Measurement. Use a ruler or a penny. Record the widest point of the crack, not an average.
5. Context. One line: “rained heavily last three days” or “new crack, was not here last month” or “door sticks only in mornings.”
6. Photo. Take one from two angles: straight on and from the side. Store them in an album named for the ADU, not scattered in your camera roll.

Do this every time you see a change. It does not have to be a perfect system. It has to be consistent enough that when you call a contractor, you can say “the crack above the west window has grown from 1/16 to 3/32 of an inch since February” instead of “there is a crack and it seems bigger.” The first sentence gets you a clear assessment. The second sentence gets you a shrug and a bid that covers the worst case because no one can verify what is actually happening.

At Fielding ADU San Jose, we ask homeowners to bring the log to the first meeting. It shortens the diagnostic process, and more importantly, it tells us whether the problem is active or stable. Active problems get a different priority than stable ones.

Common Mistakes to Avoid

• Painting over a crack before measuring it. Spackle and paint hide the evidence. A crack that is painted over and then reappears months later has now been active for months longer than you know. Measure first, then seal.
• Assuming a new ADU has no problems. New construction has defects. The first year of occupancy in a San Jose ADU is exactly when header deflection, slab moisture, and electrical load problems surface. A one-year warranty exists for a reason: that is the window when most defects announce themselves.
• Running a dehumidifier indefinitely without finding the moisture source. A dehumidifier masks the symptom. The floor is still wet underneath. Find the water, fix the water, then run the dehumidifier to dry out the residual.
• Calling a handyman for a structural issue. A handyman can patch drywall. A structural crack above a header is not a drywall problem. The patch will fail. The header will not change. You will pay twice.
• Ignoring a breaker that trips occasionally. Breakers do not wear out from tripping. They trip because the circuit is overloaded, and the overload is generating heat somewhere. An occasional trip means the circuit is running at the edge of capacity every day, not just when it trips.
• Assuming the previous owner’s contractor pulled permits. In San Jose, a buyer inherits the unpermitted work along with the property. Ask for the permit records before purchase, or if you already own, pull the permit history from the city. It is public record. It takes ten minutes on the San Jose Permit Center portal.
• Waiting until a crack is “big enough” to act on. A crack becomes more expensive to repair as it grows, not less. The same foundation issue that is $4,000 to underpin at month one can be $25,000 to remediate at month eighteen. The equilibrium point where it becomes urgent is not the point where it becomes cheap.

When to Call a Professional

Call a contractor when the log shows change over time. A crack that is stable at 1/16 inch for six months may be cosmetic, and you can wait and watch. A crack that grows from 1/16 to 1/8 inch in thirty days is active, and active means call. Call when you smell something you cannot air out, when a breaker trips more than once in a week, or when water pressure drops below what the ADU had last year. If the floor feels soft or bouncy underfoot, call that day. If the ADU is new and you are within the first year, the builder’s warranty should cover the inspection, and you should not pay out of pocket for a defect that was present at close. Fielding ADU San Jose offers free estimates in San Jose, call (669) 201-3984 and bring your log. If you are not sure whether the problem is urgent, send us the log and we will tell you what we think over the phone. That call is free too.

Frequently Asked Questions

The Bottom Line

A San Jose ADU should be dry, level, and quiet, and when it is not, the problem announces itself in specific, observable ways: the direction of a crack, the location of a stain, the sound of a drain, the trip of a breaker. Most of these signs are reversible if caught early. The ones that get expensive are the ones no one logged. Document what you see, measure what you see, be specific when you call. Look for the warning signs, and get a written answer.

Written by Reed Calloway, Owner at Fielding ADU San Jose, serving San Jose since 2006.