Building an ADU in 120 Working Days: What Los Angeles Homeowners Should Know

When you call us about an ADU, you'll ask the same two things: what's the cost and how long does it take. The price tag depends on your lot and what you have in mind. But we can give you a clearer answer on the timeline than you might expect.

Once we have your permit in hand, we can wrap up an ADU in 120 working days. That's 24 weeks of Monday-through-Friday labor, or about five and a half months total. It's not a guarantee for every property. Your square footage, the site conditions, and the finishes you pick all shift the clock. So we plan every build to hit that target on a standard site.

This post covers what those 120 days include, what we need to get done beforehand, what speeds up the job, what causes delays, and how California rules now limit fees and approval times.

Key Takeaways

• We at 1 Smart Build can wrap up ADU construction in as few as 120 working days after we pull your building permit. This depends on your specific site, the size of the build, and the finishes you choose.
• Expect the whole process, from our first site visit through design, permits, and final inspection, to take 8 to 14 months.
• California law says your ADU avoids city impact fees, special district fees, and water company fees if it stays at or below 750 square feet.
• So, if you keep your ADU or junior ADU under 500 square feet, you also skip the school impact fees.
• And once we submit a complete application, the city has 60 days to either approve or deny your project.

What "120 Working Days" Actually Covers

Our 120-day clock kicks off the moment we have your permit in hand and the crew can legally swing hammers. This timeline covers the build from start to finish. We are talking site prep, foundation work, framing, rough plumbing and electrical, every mandatory inspection, insulation, drywall, finishes, and the final sign-off.

But this count excludes design and plan check. Those phases happen first. Honestly, that is where most ADU projects hit major delays. Here is how the real timeline breaks down:

• Site visit and feasibility: 1 to 2 weeks to confirm setbacks, utilities, panel capacity and fire zone.
• Design and engineered plans: about 4 to 6 weeks.
• Plan check and corrections: 6 to 16 weeks, depending on the city and how complete the submittal is.
• Construction: as few as 120 working days after the permit is issued.

We define working days as standard business days. We do not count weekends or holidays. And we also pause the clock when heavy rain stops us from pouring concrete or laying down roofing.

Why We Can Build That Fast

Speed on an ADU isn't about rushing the crew. It's about getting rid of the waiting. Most lost time on a typical job is just waiting on a decision, a delivery, or an inspector. Here is how we handle it.

The plans are complete before they hit the city

Incomplete submittals get bounced back for corrections, and each round adds weeks. We design and build, so the same people drawing your plans are the ones building from them. We catch conflicts between structural, plumbing, and electrical before the city ever sees them.

Selections get locked early

Windows, doors, cabinets, and tile have long lead times. We finalize your selections while your plans are in plan check. That way, we order everything before the permit is issued so it’s on-site when the crew is ready.

One team runs the whole job

You aren't stuck coordinating an architect, an expediter, and a separate builder. We give you one project manager who owns the schedule. They book inspections early and keep trades moving in sequence so there aren't any gaps.

You can see where things stand

Every client gets a private portal with the schedule, milestone payments, approvals, and job-site photos. So when we need a decision from you, you see it right away instead of finding out a week late.

What Can Stretch the Timeline

We want to be upfront so you aren't surprised later. Here is what usually pushes an ADU build past 120 working days.

• Hillside or difficult sites: Soils reports, deeper foundations, retaining walls, and tight access for our equipment add time.
• Utility upgrades: We might need a new electrical service or a panel upgrade. Sometimes a new sewer connection waits on the utility company. We work around their schedules.
• Larger units: A 1,200-square-foot two-bedroom takes longer to frame and finish than a 500-square-foot studio.
• Custom finishes: If you pick materials that need special ordering, we deal with those lead times.
• Changes after construction starts: Changing a wall or window location mid-build means we stop to revise plans. You might even need a new inspection.
• Weather: Heavy rain shuts down foundation and roofing work.

We check these factors during our first site visit. You get a schedule built on your actual property, not just some best-case guess.

2026 California Rules That Help Before You Break Ground

State law makes ADUs cheaper and faster to permit now. Check the California Department of Housing and Community Development's March 2026 ADU Handbook for the details:

No impact fees at 750 square feet or less

If your ADU stays at 750 square feet or under, the city, special districts, and water companies cannot hit you with impact fees. Go over 750 square feet, and those fees must scale proportionally to your main house size.

No school fees under 500 square feet

ADUs or junior ADUs under 500 square feet skip school impact fees entirely. So, plenty of homeowners choose to keep their footprint small to save cash.

A 60-day decision on complete applications

The city has 60 days to approve or deny your permit once they mark your application as complete. This is why we make sure your submittal is perfect before we send it in. The clock doesn't start until the city accepts every document.

You will still owe standard building permit fees, plan check fees, and utility connection costs. We verify those specific numbers for your job site before we finalize any designs. You can see more general ADU info on the HCD ADU page or look up permit rules on the LADBS website.

Picking the Right Size for Speed and Cost

If you need the job done fast and want to keep your costs down, size is the main thing you control.

• Under 500 sq ft: You skip the impact fees and school fees. We can get a studio or a small one-bedroom up pretty quick.
• 500 to 750 sq ft: You still dodge the impact fees, but check for school fees. This gets you a decent one-bedroom for a tenant or family.
• Over 750 sq ft: Impact fees kick in based on the size. We are looking at a two-bedroom here, so it takes us longer to build.

And if you want to speed things up, think about a garage conversion. We use your existing slab, walls, and roof, provided the ceiling height and foundation are up to code.

How to Check Your Contractor

A fast timeline is worthless if the work isn't permitted or done correctly. Before you sign a contract, look up the license on the Contractors State License Board site. We hold CSLB license #1076428, which is a Class B General Building Contractor designation. So, grill your contractor. Ask them exactly what their timeline covers, when the clock starts, and how they handle it when a delay hits the job site.

Start With a Free Site Visit

Your ADU build starts right on your lot. We'll walk your property to see what's actually buildable. We'll spot issues that could hold things up, then give you a realistic timeline and a fixed-price proposal. So, why not run a quick check on your property using our free build check? You can also read up on our ADU and garage conversion services here.

Call 1 (866) 419-8840 to set up your free consultation.