ADUs: what "by right" really means before you build one

01An ADU is a full second home, not a spare room

An accessory dwelling unit, or ADU, is a complete second home on a lot that already has one. It has its own kitchen, its own bathroom and its own entrance. A bedroom with a mini-fridge does not count; separateness is the whole idea.

People build them for very different reasons. A parent moves in. An adult child moves back. The unit gets rented for income, or it waits empty as future flexibility. The reason you are building changes what a good outcome even looks like.

An ADU is not a good idea or a bad idea. It is a question about your lot, your money and your timeline.

02"By right" means yes with conditions, not no questions asked

By right means a use is allowed without a discretionary vote. You still pull permits and still meet every written standard, but a board cannot simply say no because a room filled up with objections.

The opposite is a discretionary permit, where a board weighs your project and can attach conditions or deny it outright. By right is faster and far more predictable, and that predictability is worth real money.

03Size and parking limits decide whether one actually fits

Most rules cap how big an ADU can be. The ceiling is often tied to the size of the main house or to a flat square-foot limit, whichever is smaller. Bigger is not automatically allowed, and it is rarely cheaper.

Parking is the quiet dealbreaker. Some places require an added off-street space for the new unit. On a tight lot, one required space can end a project before design even starts.

Setbacks, height and lot coverage push from the other direction. They decide where a unit can sit, not just how large it can be. Wiew reads lots most deeply in Massachusetts today, so the buildable picture is richest there.

04Detached, attached and interior are three different projects

There are three broad forms, and they are really three different projects.

• Interior. Carved from existing space, like a basement or a room over the garage. Usually the least expensive, because the shell is already standing.
• Attached. Shares a wall with the main house, like an addition or a converted garage bay. Middle of the pack on cost and complexity.
• Detached. A standalone building in the yard. Usually the most expensive, foundation to roof, and often held to the tightest setback and coverage limits.

Utilities follow the form. A detached unit may need its own service runs, while an interior one can sometimes tap what is already there. None of this shows up in the floor plan you fall for.

05Overlays and special permits are the exceptions to by right

An overlay district is an extra layer of rules mapped on top of the base zoning for a specific area. It can loosen limits or add them. Two lots on the same street can play by different rules if the boundary runs between them.

A special permit is the discretionary path. You apply, a board reviews, and approval is never guaranteed. It exists for projects that step outside the by-right box, say a unit over the size cap or one short on parking.

The practical move is to learn whether your address is by right or special-permit before you spend on drawings. The same ADU can be routine on one lot and a months-long hearing on the next.

06Financing an ADU is about your whole picture, not the unit

An ADU is a serious build, and most people borrow to do it. Common routes lean on the equity in the main home or fold the cost into a renovation loan. Who qualifies, and on what terms, varies widely, so this is a conversation for a lender, not a rule of thumb.

Lenders look at your income, your existing debt and the home's value, not the ADU on its own. Expected rent from the future unit may or may not count toward what you can borrow, so do not bank on it.

Weigh what the unit gives back, too. Added value is a range, not a promise, and it turns on location, form and the local rental market. A modeled band shows roughly where you stand; it does not guarantee a payout.