Can you build an ADU? Find out in seconds.

ADU eligibility depends on your zoning designation, lot size, state law, and local ordinances. Several states (California, Oregon, Washington, Colorado, Vermont, Maine, Connecticut, Montana, New Hampshire) now require municipalities to allow ADUs by-right on residential lots. Enter your address above for a state-specific eligibility check.

An ADU (Accessory Dwelling Unit) is a secondary housing unit on a single-family residential lot. Also called granny flats, in-law suites, or backyard cottages. ADUs can be detached, attached, converted from existing space like a garage, or built within the primary home as a Junior ADU (JADU).

ADU construction costs typically range from $150,000-$350,000+ depending on size, location, and whether it is detached new construction, attached addition, or garage conversion. Prefab ADUs can start around $100,000-$200,000. Garage conversions are the most affordable option at $90,000-$180,000. Building in a flood zone adds $15,000-$50,000 for elevation requirements.

Statutes in force as of 2026: California (AB 68), Oregon (SB 1051), Washington (HB 1337), Colorado (HB 24-1152), Vermont (Act 47), Maine (LD 2003), Connecticut (PA 21-29), Montana (SB 528), and New Hampshire (RSA 674:71). Massachusetts (Aug 2024) allows by-right on most single-family lots. Arizona (HB 2720, effective 2025) mandates ADU allowances in cities ≥75,000. Confirm the current version of each statute with your state legislature before relying on a specific provision.

California stacks four statutes on a single residential parcel. Baseline (AB 68/881/SB 13): 1 primary + 1 ADU + 1 JADU = 3 units. SB 9 duplex by-right adds a second primary unit → 4 units. SB 9 urban lot split (lot ≥2,400 sqft, 60/40 split max) bisects the parcel — each new lot can hold a duplex + ADU, taking the stack to 6 units. SB 1123 (effective July 1, 2025) extends ministerial lot splits to vacant single-family lots in metropolitan counties and removes the SB 9 owner-occupancy affidavit, enabling up to 10 units. Lot-split rights are blocked in SFHA, very-high fire severity zones, prime farmland, wetlands, and historic districts. The free checker computes the maximum stack for your specific parcel.

Maximum ADU size varies by state: California 1,200 sqft detached, Oregon 900 sqft or 75% of primary, Washington 1,000 sqft, Colorado 1,000 sqft. Most other states default to 800-1,200 sqft. JADUs are typically capped at 500 sqft within the existing home footprint.

In California (AB 670) and Oregon, HOAs cannot unreasonably restrict ADU construction on single-family lots. Other state mandate laws do not explicitly override HOA covenants. Outside mandate states, HOAs typically retain veto power. Check your CC&Rs and state law.

Owner-occupancy requirements have been eliminated in most ADU-mandate states. California, Oregon, Washington, Colorado, Vermont, Maine, Connecticut, Montana, and Massachusetts (2024) do not require owner-occupancy. New Hampshire still permits towns to require it. Most local-control states still allow owner-occupancy mandates.

Total ADU timeline from design to occupancy is typically 12-18 months. Permitting takes 2-6 months depending on jurisdiction (60 days max in California). Construction takes 6-12 months for detached ADUs, 4-8 months for garage conversions, and 3-6 months for prefab units.

Yes. In most jurisdictions, ADUs can be rented as long-term rentals. Some cities also allow short-term rentals (Airbnb), but many states (CA, OR) restrict ADU short-term use. Rental income from ADUs typically ranges from $1,000-$3,000/month depending on market and size.

Yes. We pull live parcel data (zoning code, lot size, existing structure size) from county records via Regrid, FEMA flood zone data from official sources, and apply state-specific ADU law to your actual address. The verdict reflects your specific lot — not a generic answer.