What an ADU Actually Costs — And the Expenses Nobody Quotes You

Most ADU projects do not go over budget because the construction was underpriced. They go over because the estimate covered the building and not the things the building has to connect to — the electrical panel, the sewer line, the driveway the concrete truck cannot reach. Those items are invisible on a quote that only lists square footage, and they are the ones that move the final number.

This guide is about the decisions that set your budget before anyone pours a slab: what type of ADU your lot can actually take, what genuinely drives the price, the costs people find out about halfway through, and how the timeline really breaks down in California versus Texas.

First, which kind of ADU are we talking about?

The word covers four quite different projects, and they do not cost remotely the same:

• Garage conversion — converting an existing attached or detached garage into living space. The structure and slab already exist, which is why this is usually the most accessible route.
• Attached ADU — an addition that shares a wall with the main house. Shorter utility runs, but it entangles the new work with the existing structure.
• Detached new construction — a standalone unit in the yard. The most freedom in design, the most expensive per square foot, because everything starts from bare ground.
• Junior ADU (JADU) — a small unit carved out of the existing house, typically with its own entrance and a limited kitchen. The cheapest option and the most restricted.

People usually arrive with a shape in mind and adjust once they see what the lot allows. Setbacks, existing utility locations and access for equipment tend to decide this more than preference does.

Garage conversion vs. building new

The garage looks like the obvious saving, and often it is. But an existing structure comes with existing conditions, and some of them cost more to fix than to build fresh.

Factor	Garage conversion	Detached new build
Structure and slab	Already there — but must be verified	Built to spec from scratch
Slab condition	Often unrated for living space; may need insulation, moisture barrier or replacement	Designed correctly the first time
Ceiling height	Fixed — and sometimes below code once flooring goes in	Your choice
Insulation and weatherproofing	Usually absent; garages are not built as conditioned space	Included in the build
Layout freedom	Constrained by existing walls and door openings	Open
Parking	You lose a space, which some jurisdictions still care about	Unaffected
Typical timeline	Shorter	Longer

The honest test is whether the existing garage is sound. A well-built structure on a level, dry slab is a real head start. A sagging structure on a cracked slab is a demolition you are paying to work around.

What actually drives the cost

Any figure quoted without seeing your property is a guess, and you should treat it as one. What is consistent is the list of things that move the number:

• Foundation and site work — slope, soil, drainage and how far the unit sits from the street
• Utility runs — distance from the main house to the new unit for water, sewer, gas and electrical, and whether those trenches cross hardscape you will have to restore
• Electrical service capacity — covered below, because it surprises people
• Sewer connection — whether the existing lateral can take another unit, or you need a new connection
• Kitchen and bathroom specification — the single biggest swing under your control
• Access — if a concrete truck, lumber delivery or excavator cannot reach the back yard, materials get moved by hand, and labor hours climb accordingly
• Permits, plan check and impact fees — these vary enormously between jurisdictions

Ask for an estimate that breaks these out as separate lines. A single lump sum makes it impossible to compare two bids honestly, and impossible to know what happens when something is discovered underground.

The costs people find out about late

These are the items that turn a comfortable budget into a tight one. None of them are exotic; they are simply easy to leave off a quick quote.

Electrical panel capacity

A second dwelling adds a full set of loads — range, water heater, HVAC, laundry. Many older homes in Pasadena and Los Angeles are running panels that were adequate for one household and are not for two. A service upgrade is a meaningful line item, and it involves the utility, which means it runs on the utility's schedule rather than yours.

Sewer capacity and the lateral

The pipe from your house to the main was sized for one home. Adding a unit can require upsizing it, and if that line runs under a driveway or mature landscaping, the restoration costs more than the pipe.

Fire safety requirements

Depending on the jurisdiction, the size of the unit and whether the main house has sprinklers, a new ADU may trigger fire sprinkler requirements. That is not just the sprinklers — it can mean a larger water service line.

Impact and connection fees

Separate from permit fees, some jurisdictions charge for the added burden on water, sewer and schools. Rules around when these apply to ADUs have changed repeatedly, so this is worth confirming specifically for your address rather than assuming.

Insurance and property tax

An ADU changes what you have insured and typically adds assessed value. Neither is a construction cost, but both are ongoing, and they belong in the arithmetic if you are running the numbers as an investment.

How long it really takes

Homeowners tend to picture the build and forget the two phases either side of it. In practice a project runs through:

1. Feasibility and design — measuring what exists, confirming what the lot allows, producing drawings
2. Plan check and permitting — submitting to the local agency, answering their corrections, resubmitting. Corrections are normal, not a sign something went wrong
3. Utility coordination — runs in parallel where possible, but a panel upgrade or new meter moves at the utility's pace
4. Construction — with inspections at defined stages that must be passed before work continues
5. Final sign-off — certificate of occupancy before anyone can legally move in

Permitting timelines vary widely by city and by how complete the initial submission is. The single biggest accelerator is a drawing set that anticipates the reviewer's questions; the single biggest delay is discovering a site condition after approval that forces a redesign.

California and Texas are not the same conversation

In California, ADUs sit inside a statewide framework that has been progressively loosened over the past several years, limiting how far local agencies can restrict them and setting deadlines for review. The practical effect is that in most California cities the question is less whether you may build one and more what the site requires. The specifics have changed repeatedly, so confirm current requirements with your local planning department rather than relying on what was true a couple of years ago.

In Texas, there is no equivalent statewide framework. Rules are set city by city, and zoning, lot size and deed restrictions carry more weight. In the Dallas and Plano area, HOA covenants in particular decide a great many projects before the city ever weighs in — that is worth checking on day one, not day thirty.

Five mistakes that cost real money

1. Designing before checking feasibility. Drawings for a unit the setbacks will not allow are drawings you pay for twice.
2. Budgeting the building and not the connections. Panel, sewer, water and trenching are the usual gap between estimate and reality.
3. Assuming the garage slab is fine. It was poured to park a car on, not to live on. Sometimes it is fine. It should be checked, not assumed.
4. Ignoring access. A narrow side yard changes how every material reaches the site, and labor is the largest single cost in most builds.
5. Treating rental income as certain. If the project only works at an optimistic rent, it does not really work. Run it at a conservative number and see if you still like it.

Where to start

Start with the lot, not the layout. Setbacks, panel capacity, sewer and access will tell you which of the four ADU types is realistic for your property, and that decision constrains everything downstream. Once you know what the site supports, the design and the budget stop being guesswork.

If you want that assessed properly, we do it on site and tell you what we find — including when the honest answer is that a garage conversion makes more sense than the detached unit you had in mind.