What Is an Accessory Dwelling Unit? A Complete Guide

The phrase self-contained does the legal work. A unit qualifies when it has independent facilities for sleeping, cooking, and sanitation in one structure. A finished basement with a bedroom and bathroom but no kitchen does not make the cut. A garage converted with a full kitchen, bathroom, and sleeping area does. Permit path, occupancy, and financing all flow from whether the project produces a true independent dwelling.

A junior accessory dwelling unit, or JADU, is a smaller and less independent version of the same idea. In California, a JADU is capped at 500 square feet and must be carved out of the existing single-family house, including an attached garage. It may share a bathroom with the main house and typically needs an efficiency kitchen rather than a full one. Standard ADUs have fully separate facilities. JADUs have more flexibility on the bathroom and stricter rules on location and size. Confirm which category you are in before you talk to a contractor.

The type you can build is usually a zoning question first and a design question second. Privacy, cost, and construction method follow from that.

Rental income is the most common driver. A well-placed secondary suite in a high-cost market such as California, Washington, or Texas can offset a meaningful share of the mortgage. Size, parking, and transit access change what renters will pay. Check local rents before you freeze the floor plan.

For many families the motivation is not income. An aging parent who wants independence, or an adult child saving for a first house, is better served by a private entrance, kitchen, and bath than by a spare bedroom. A growing share of owners also build a dedicated office or studio and keep the option to rent later. A permitted secondary unit is a long-term property asset even if year-one use is not a lease.

The first question planning staff will answer is whether your lot is eligible, and for which type. A growing majority of US single-family zones now allow at least one accessory unit, but the details are local.

California has the most detailed statewide framework. HCD's ADU handbook describes a ministerial path for complete applications, common four-foot rear and side setbacks for many detached units, and a protected option of at least 800 square feet when objective standards are met. Local ordinances cannot set a maximum below 850 square feet for a studio or one-bedroom ADU, or 1,000 square feet for a larger unit. Cities cannot require parking in several statutory cases, including units within a half-mile walk of public transit and conversions inside an existing house or accessory structure. Where parking is still allowed, the cap is one space per unit or bedroom, whichever is less. That is not a blanket parking ban. Confirm the exemptions that apply to your lot.

Massachusetts is no longer a purely local-ordinance state. The Affordable Homes Act, signed in August 2024, allows by-right ADUs up to 900 square feet in single-family zoning districts. State regulations took effect on January 31, 2025. Towns can still apply dimensional rules, but they cannot require a special permit for a protected-use ADU. New York, Colorado, and most other states still leave more of the map to city and county codes.

The permit sequence is consistent even when the statute is not: confirm zoning, prepare site plans, submit a complete packet, answer plan-check comments, pay fees, and pass inspections. Plan review alone often takes four to twelve weeks. Incomplete documents and setback or lot-coverage conflicts are the usual stall points. A designer or contractor who has already worked with that building department is worth the extra call.

Nationally, building an ADU often runs about $150,000 to $400,000 all-in, with many mid-size projects on simpler lots landing around $180,000 to $280,000. Garage conversions and internal conversions sit toward the lower end because the structure exists. Detached new builds cost the most: foundation, framing, utilities, and finishes all start from zero.

Factory-built units often beat the top of that range on the box itself. They do not automatically beat it on the project. A 400-square-foot studio can still land near $160,000 to $230,000 installed once foundation, utilities, delivery, and permits are in. Stick-built detached work on the same lot is often higher, but a cheap volumetric quote on a steep yard with a long sewer run can finish more expensive than a site-built unit on a flat, utility-ready lot. Compare fully installed, permitted, connected prices. The factory vs turnkey bands are in the prefab ADU cost section. Site and utility surprises belong in the same conversation as what to expect beyond the factory sticker.

Foundation and site work commonly run about $5 to $40 per square foot depending on soil, slope, and access. Sloped, rocky, or poorly drained lots hit the upper end. On a simple suburban lot, a grade-plus-foundation-plus-short-trench bundle often lands around $15,000 to $45,000 before the unit is ordered.

Utilities, electrical, and HVAC together often account for 20 to 25 percent of total project cost. New lines from the house or the street are the line item first-time ADU owners underestimate. Give it its own number on day one, not a leftover after you pick finishes.

Kitchen and bathroom packages are the largest interior costs because they combine rough plumbing, fixtures, cabinetry, and finishes. Simple layouts and mid-range fixtures are the most reliable way to keep livability without letting the interior absorb the whole contingency.

If you have equity in the primary home, a HELOC or a home equity loan is the usual starting point. A HELOC lets you draw as construction progresses. A home equity loan is a fixed lump sum. A cash-out refinance replaces the existing mortgage with a larger one, which only helps if the new rate is acceptable. For many owners with substantial equity, one of those three tools funds the build without a separate construction product.

If equity is thin, a construction-to-permanent loan funds the work in stages and converts to a longer mortgage once the ADU is complete. Renovation loans such as Fannie Mae HomeStyle or FHA 203(k) can underwrite against post-improvement value. Prefably does not broker loans. If the unit is factory-built, read How to get a mortgage for a factory-built home and modular home financing before you treat a catalog price as financeable.

Public programs change faster than blog posts. CalHFA's ADU grant offered up to $40,000 for pre-development costs. The latest round was fully allocated on December 28, 2023, and CalHFA is not accepting new applications. Treat anyone who claims they can still get you that grant as a scam. In Massachusetts, MassHousing launched an ADU construction loan in March 2026: up to $250,000 for detached units and $150,000 for attached units, as a fixed-rate second mortgage for income-eligible owner-occupants who already have permits. Details and lenders are on MassHousing's ADU page. Cities and counties layer their own fee waivers and technical-assistance programs on top. Check the local housing authority before you freeze the capital stack. Santa Cruz County, for example, replaced an older forgivable-loan offer with a technical-assistance program, which is why you should not budget against a city grant you have not confirmed this month.

An accessory dwelling unit can produce income, house family, and add long-term property value. The useful question is which type fits this lot, this budget, and this code path.

Start with the local zoning office. Confirm eligibility, size, and setbacks. Build a budget that includes site work, utilities, and permits, not just the unit. Match financing to your equity and to programs that are actually open.

If a factory-built or modular unit is on the list, Prefably makes it straightforward to find ADU-capable builders in your state, read their profiles, and reach out yourself. No lead-selling. No sales calls you did not ask for. Browse builders by state, then use What is a prefab ADU when you are ready to compare kit, modular, and HUD-code backyard products.