An accessory dwelling unit is a small, self-contained home sharing a residential lot with a primary residence: its own kitchen, bath, and entrance, on the same parcel as the main house. ADUs take several physical forms — a detached cottage, an attached addition, a garage conversion, a basement apartment — and jurisdictions track them in exactly those buckets. What varies by place are the rules around them: size caps, occupancy terms, parking, fees, and permit paths all answer to local ordinance, which is why this guide teaches the check-process rather than quoting any single code. If you are converting a garage specifically, our garage conversion method owns the construction procedure; this page defines the territory.

At Nuvira Space, we treat the ADU as the most consequential small building in housing: a second home that can keep family close, fund a mortgage, or gentle-densify a street without changing its character. Its difficulty is never construction complexity but permission clarity — the same structure that is by-right in one city needs discretionary approval in the next. This guide therefore defines the unit, maps its forms, separates it from look-alikes, and gives you the exact sequence of local checks to run before spending on design. Procedure for conversions, use-cases for multigenerational living, and the missing-middle context live in our spokes; this page makes sure you are pointing at the right thing before you open any of them. It carries no costs, no product links, and no code citations — those belong to the cost guide, the product page, and your local ordinance respectively, each reached from here but never replaced by here. Definitions first, always.

ADU types: the verified forms

Tracking systems and housing agencies converge on three buckets — attached, detached, and internal — with conversions as the practical route into two of them. Massachusetts describes the same span plainly: inside an existing home like a basement apartment, attached as an addition, or fully detached like a cottage or converted garage.

Type	What it is	Typical fit
Detached	Standalone cottage or new backyard structure with full services	Lots with yard depth and separate-access potential
Attached addition	New living space joined to the primary residence	Homes where an extension reads naturally on the plan
Garage conversion	Existing garage rebuilt as a dwelling interior	Properties with underused parking structures; procedure in our conversion guide
Internal conversion	Basement or underused interior area partitioned as a unit	Homes with separable floor area and compliant egress potential

ADU versus tiny home versus addition

Three confusions waste the most homeowner time, so settle them before anything else. A tiny home is a size and often mobility category, while an ADU is a legal relationship to a lot and a primary dwelling — a tiny home sited as a second unit answers to ADU rules, not to tiny-home branding. An addition expands the primary dwelling itself, while an ADU remains a separate dwelling with its own kitchen, bath, and entrance even when physically attached. And a guest suite without independent cooking and sanitation is not a dwelling unit at all under most ordinances — self-containment is the bright line that makes an ADU.

Option	What defines it	What rules apply
ADU	Second self-contained dwelling on the lot	ADU ordinance: size, occupancy, parking, fees, permit path
Tiny home	Small, often movable structure	Whichever rules cover its siting — as an ADU if second unit, otherwise trailer, park, or primary rules
Addition	Expansion of the primary dwelling itself	Standard residential addition permits; no second-unit rules
Guest suite	Bedroom space without independent cooking	Interior remodel rules; not a dwelling unit

Regulation check: the process, not the code

No national ADU code exists to quote — rules answer to states, counties, and cities, and they move every legislative cycle. Massachusetts illustrates the shape: a statewide by-right allowance with municipalities still empowered over site review, setbacks, short-term rentals, and utility rules, plus evolving draft regulations under public comment through 2026. Your city will rhyme with this pattern without matching it. Run the checks in order and in writing before spending on design.

Confirm the local ADU ordinance. Ask planning whether ADUs are permitted on your lot and zoning class, and which document governs — ordinance number and date, not hearsay.
Read the occupancy and use terms. Owner-occupancy mandates, family-relationship rules, and short-term rental prohibitions vary widely and decide the business case.
Clear parking, fees, and setbacks. Parking minimums near transit, impact and connection fees, and dimensional setbacks are the most common surprise costs and design constraints.
Verify utility and sewer capacity. Water, sewer or septic, and electrical service must be confirmed with the serving agencies — Title 5 style septic rules alone can decide feasibility.
Map the permit path. By-right administrative approval versus discretionary review changes timeline, cost, and risk; get the path named before you hire.

Where an ADU earns its place

Three use-cases carry most real projects into existence. Multigenerational households use ADUs to keep family close with autonomy intact — see our intergenerational suites method for the design procedure. Owners fund mortgages or retirement with long-term rental income from a unit the lot already has room to hold. And neighborhoods absorb gentle density without changing character, the missing-middle pattern our missing-middle housing guide places in its economic and zoning context. Construction figures belong to our ADU cost guide; figures are deliberately absent here.

First decisions before design

Three answers sequence everything downstream. First, lot fit: setbacks, access, and utility capacity decide which types are physically possible before taste enters the discussion. Second, rule fit: the checklist above decides which possible types are legally buildable and under what terms. Third, use fit: rental income, family housing, or future flexibility decides which buildable type deserves the budget. Any type failing an earlier answer drops out — do not design around a veto you have not checked. When a lot fails all three, waiting for ordinance reform beats building noncompliant square footage you cannot permit, insure, or sell.

Frequently asked questions

Q: What exactly counts as an ADU?

A: A small, self-contained home on the same lot as a primary residence, with its own kitchen, bath, and entrance. Self-containment is the bright line: without independent cooking and sanitation, it is a suite, not a dwelling unit.

Q: Is a tiny home automatically an ADU?

A: No. Tiny describes size and often mobility; ADU describes a legal relationship to a lot and a primary dwelling. A tiny home sited as a second unit answers to ADU rules in that jurisdiction.

Q: Do ADU rules differ by location?

A: Completely. Size caps, occupancy terms, parking, fees, and permit paths all answer to state and local ordinance and move every legislative cycle. Run the regulation checklist above against your own city before spending on design.

Q: Garage conversion or detached cottage — which first?

A: Conversions reuse structure and services and usually clear feasibility fastest; detached units offer autonomy and rental separation at higher scope. Our conversion method details the garage path specifically.

Q: Where do costs get covered?

A: In our forthcoming ADU cost guide, which carries sourced 2026 figures. This page deliberately holds no cost numbers — construction economics decay by market and belong with verified data, not definitions.

Q: Can an ADU work for aging parents?

A: Yes — single-level detached or converted units near the primary home are the classic pattern, with autonomy plus proximity. Our intergenerational suites method details the design; confirm occupancy and caregiver terms in the regulation checklist first.

ADU directory

Build it in our garage conversion method; house it multigenerationally via our intergenerational suites; place it in policy context with our missing-middle housing guide.

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