Can you build an in-law apartment?

Zoning decides whether, building decides how

An in-law apartment — an accessory dwelling unit, in the language the rules use — is a second, self-contained home on a property that is zoned for one. That is why it is a zoning matter before it is anything else.

Once a town has said yes and on what terms, the construction is ordinary: framing, plumbing, electrical, insulation, a kitchen, a bathroom, a way out in a fire, separation between the units. None of that is unusual work. It is the permission that carries the uncertainty, and the permission is what sets the size, the entrance, the parking and sometimes whether it is possible at all.

So the sequence is always the same. Find out what the town allows at your address. Then design to that. Designing first and asking afterwards is how people end up paying twice for drawings.

It is the permission that carries the uncertainty, not the construction.

The questions a town will ask

Where the unit is

Inside the existing house, attached as an extension, or detached in the garden. Towns treat those three very differently, and detached is usually the one with the most conditions.

How big it may be

Normally expressed relative to the main house or as a cap. It is the constraint that most often decides the layout.

Its entrance

Whether it may have its own door, where that door may be, and whether it may face the street. Appearance from the road matters to a lot of ordinances.

Parking

Whether an additional space is required, and where it may go without eating the front garden.

Who may live in it

Some ordinances attach conditions about occupancy or about the owner living on the property. Whether any apply to you is a question for the town.

Whether the services can take it

Particularly waste. On a septic system, a second kitchen and bathroom is a load question, and the system's design may have to change before anything else can.

Three ways to make one

1. Convert space you already have

   A basement, an attic, a wing, part of a large ground floor. Usually the least disruptive route, and it lands squarely on the habitable-space tests: headroom, a way out in a fire, light and ventilation, moisture, insulation.

2. Add on

   An extension built for the purpose. This brings the full zoning picture with it — setbacks, lot coverage, height — because you are making the building bigger as well as making it two homes.

3. Convert or build detached

   A garage, an outbuilding, or something new in the garden. Often the most attractive option and usually the most heavily conditioned: it needs its own services run to it, and towns look hardest at this one.

Septic and water are the quiet blockers

Of everything on this page, the item most likely to stop a project without warning is waste.

A property on town sewer generally has room to manoeuvre. A property on an on-site system does not automatically: the system was designed for a specific load, and that design is the thing a town checks when you propose adding a second kitchen and a second bathroom. Where the existing system cannot carry it, the answer is a new or enlarged system, which is its own designed, approved and permitted project with its own cost and its own timetable.

The same logic applies more mildly to water supply and to the electrical panel. None of it is a reason not to proceed. All of it is a reason to ask in the first week rather than the fifth month.

The rules here have been moving

This is one of the few areas of building where what was true a while ago may simply not be true now.

Both state law and individual town ordinances around accessory units have been revisited more than once in recent years, in the direction of making them easier in some circumstances and leaving conditions in place in others. The practical consequence is that a neighbour's experience, a forum post or an article is not a reliable guide to what you may do at your address today.

So check the current version, for your town, before you commit to anything. It is a short conversation with the town or with someone who has that conversation regularly, and it saves the expensive kind of surprise.

If it is for someone who cannot manage stairs

Say so at the start, because it changes the design from the first line rather than at the end.

A unit intended for a parent or a relative with limited mobility wants a level entry or a ramp that works in ice, door widths that take a walker or a chair, a bathroom laid out for the turn as much as for the fixtures, and the essentials on one floor. Those are ordinary things to build and awkward things to retrofit, and several of them interact: a wider door is no use if the turn into it is too tight.

If there is a date attached — someone coming home from hospital, a house being sold — say that too. It changes the order of everything.