How Long Does It Actually Take to Build an ADU in NYC? A Realistic Timeline
One of the first questions homeowners ask when they start thinking about an ADU is: "How long is this going to take?"
It's a fair question. And the answer depends on a few things — but here's a realistic breakdown of what to expect.
The Short Answer: 6 to 12 Months After Permit Approval
At Second Key, we say "6 months or less from permit to move-in" — and we mean it. That's for construction. But there are a few steps that happen before construction even starts, and those add time to the overall timeline.
Keep reading for the full timeline, broken down into five phases.
Phase 1: Feasibility (2–3 weeks)
This is where you figure out whether an ADU is even possible on your property. At Second Key, we offer a free initial assessment that checks zoning rules, building code, and flood zones. This happens fast — usually within a few hours of reaching out. Once you know an ADU is possible, the recommended next step is to perform a comprehensive feasibility study to show you the full picture of what can be built and how much it will cost. Read more about our comprehensive feasibility studies here.
Phase 2: Design & Planning (4–8 weeks)
After feasibility, the next step is designing your ADU. In NYC, all ADUs require the preparation of full architectural plans and engineering plans, which can take 4-8 weeks. If you’re using a pre-approved ADU plan from NYC’s Pre-approved Plan Library, this phase is faster. The Pre-approved Plan Library is expected to go live in mid-to-late 2026.
Phase 3: Permitting (3–6 months)
This is the longest and most unpredictable part of the process. Your architect files the plans with the NYC Department of Buildings through DOB NOW: Build. The DOB reviews the plans, asks for revisions, and eventually issues a permit.
The timeline depends on the complexity of your project and how backed up the DOB is. Simple projects with pre-approved plans can move faster. Custom designs with unusual site conditions can take longer.
Phase 4: Construction (2–6 months)
Once permits are in hand, construction begins. For most ADU projects in NYC, construction takes 2–6 months. Basement conversions tend to be on the faster end. New detached structures take longer.
Phase 5: Inspections & Certificate of Occupancy (2–4 weeks)
After construction is done, the DOB needs to inspect the unit and issue a Certificate of Occupancy (CO). This process usually takes a few weeks, assuming everything passes inspection the first time.
Total Timeline: 7–18 Months (Start to Finish)
If you're starting from scratch, plan on about a year from your initial consultation to the day someone moves into the ADU. That includes feasibility, design, permitting, construction, and inspections.
Can You Speed It Up?
A few things can shorten the timeline:
Working with an experienced ADU builder (like Second Key) who will streamline the process and assemble the right team of design professionals and permit expeditors
Having a straightforward lot with no complex zoning issues or structural complications
Being responsive when the DOB asks for revisions or additional info
The Bottom Line
Building an ADU in NYC takes time — but it's not an endless process. With the right team, realistic expectations, and a clear plan, you can go from "just an idea" to "keys in hand" in less than a year.
Ready to Get Started?
The first step is a free assessment. We'll tell you what's possible, estimated cost, and how long it will realistically take.