HyreADU

Step 09 · Build

How an ADU actually gets built

The sequence is not complicated. What varies is how long each phase takes, and almost all of that variance sits in two places: permitting and utilities.

The short answer

ADU construction runs in five phases: site preparation and foundation, framing and envelope, rough mechanical/electrical/plumbing, interior finishes, and final inspections and occupancy. Design and permitting come before all of it and typically take longer than owners expect. Measured permit-to-completion durations from California records are published in our research.

What moves it

The five phases

What each one is waiting on:

  1. 1Site preparation and foundation

    Clearing, excavation, utility trenching and the pour. Weather and soils both show up here.

  2. 2Framing and envelope

    Structure, roof, windows and weatherproofing. The most visible phase and often the fastest.

  3. 3Rough MEP

    Electrical, plumbing and mechanical before walls close, followed by inspection. Nothing proceeds until the inspection passes.

  4. 4Interior finishes

    Insulation, drywall, kitchen, bath, flooring. The phase most exposed to material lead times and to change orders.

  5. 5Final inspections and occupancy

    Sign-offs across trades, then the certificate. Utility connection scheduling frequently gates this.

01

What actually controls the schedule

Two things, and neither is framing. The first is permitting, which is outside your control and your builder’s — see the permits hub for the statutory clocks and the measured record.

The second is utility connection. Scheduling a service connection or a sewer tie-in depends on the utility’s calendar, and it can sit on the critical path at the end of a project when everything else is finished.

Our timeline planner asks you to enter each phase separately and sums them, precisely because a single headline duration hides which phase is the risk on your project.

02

On promised completion dates

We do not publish a typical ADU build time as though it were a commitment, and you should treat any site that does with caution.

What we publish is the measured distribution of permit-to-completion durations from California jurisdiction records, with its limits stated. That is a different claim from a promise, and it is the honest one.

03

Change orders

The most common cause of an ADU going over budget is not the original price. It is what is discovered once work starts — a slab that fails, a service that cannot carry the load, a septic system at capacity.

The defense is diligence before signing: the site conditions established early, and a contract that says how a change order is priced and approved before one is needed.

Questions

Questions people ask

More in the full FAQ.

How long does it take to build an ADU?

Construction itself is usually a matter of months, but design and permitting come first and frequently take as long again. We publish measured permit-to-completion durations from California records rather than a headline figure; see our construction duration research for the distribution and its limits.

Does prefab build faster?

It compresses the on-site phase, because the unit is being built while site work proceeds. It does not remove the site work, the foundation, the utility connections or the permitting, and those are where most of the schedule variance lives.

Can I live in the main house during construction?

Usually, yes — that is one of the practical advantages of a detached ADU over a whole-house remodel. Expect disruption around utility tie-ins, when existing services may be interrupted.

Sources

  1. HyreADU research: ADU construction duration (California jurisdiction permit records) Retrieved 2026-09-05

Last reviewed 2026-09-16

Plan your project

Tell us about your ADU

Three short steps: the ADU you want, your home, and how to reach you. Nothing goes to a contractor unless you agree.

A straight answer: our ADU professional matching is still being built, so this is not a quote service, and we cannot tell you whether your lot qualifies. We can help you work out which questions to ask and what a thorough proposal should contain. Your planning department decides what is permitted at your address.

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Tell us about the ADU you are planning and where you are. We will follow up about connecting you with contractors serving your area.

Step 1 of 3: your project