What an accessory dwelling unit is
An accessory dwelling unit, often shortened to ADU, is a small, self-contained home on the same lot as a larger primary house. It has its own kitchen, bathroom and sleeping area and its own entrance. The usual forms are:
- Detached units, such as a backyard cottage or a converted outbuilding.
- Attached units, such as an addition built onto the side or back of a house.
- Internal units, such as a finished basement, attic or converted garage apartment.
Terms vary by place. People also say granny flat, in-law suite, laneway house or secondary suite. The common idea is a second, smaller home that shares a lot with the first.
Why cities pay attention to them
Infill means building on land that is already within the developed area instead of at the edge. Most infill discussion focuses on vacant lots or larger redevelopment sites. ADUs are infill at the smallest scale: they use space already owned by a household, on streets that already have pipes, wires and roads.
Several things make them appealing to planners and to some residents:
- They add homes without changing the basic street pattern or requiring large sites.
- They can house relatives, such as an aging parent or an adult child, close by but with privacy.
- They can give a homeowner rental income, which some households use to afford their own mortgage.
- They tend to be smaller and so are often cheaper to rent than a full house, although not necessarily cheap.
Some people see them as a gentler path to more homes in neighborhoods where larger buildings face strong opposition. The wider argument over what such neighborhoods should allow is covered in the debate over single-family zoning.
What zoning rules typically decide
Whether an owner can build one depends on local rules, which differ widely. A zoning code usually answers several questions:
- Is an ADU allowed at all in the district, and by right or only through a special approval?
- How big may it be? Many codes set a maximum floor area, or a fraction of the main house’s size.
- Where may it sit? Setback rules, height limits and lot coverage still apply, so a small lot may have no legal room for a detached unit.
- Must the owner live on the property? Some places require it; others do not.
- How much parking is required? Parking requirements have been a common barrier, and parking minimums are often loosened for ADUs.
- May it be rented short term? Some cities restrict that use to keep the units for longer-term housing.
Building and fire codes apply too, covering egress, smoke alarms, ceiling heights and similar basics. These often matter more for basement and garage conversions than for new construction.
Practical limits
ADUs are modest by nature, and several practical factors keep them from changing housing supply quickly.
Cost. Building even a small separate home involves site work, utility connections, design, permits and construction. Costs vary by region and are rarely trivial. Financing is also awkward, since many owners must borrow against their existing home, and not every household can or wants to.
Utilities. A new unit may need its own or upgraded water, sewer and electric connections. Where infrastructure capacity is tight, this can add expense or delay.
Lot constraints. Many lots are too small, too steep or too crowded with existing structures. Sewer lines, trees and easements can also block a good location.
Owner choice. Each ADU requires a homeowner who decides to build one. Unlike a developer, a household may have no interest in becoming a landlord, so even permissive rules often produce a trickle rather than a wave. How many get built in a given city depends on local costs, rules and demand, and the numbers are hard to predict.
Fees and process. Impact fees, permit approvals and design requirements can erase much of the advantage of a small project. Cities that want more ADUs sometimes publish pre-reviewed plans or reduce fees to ease this.
Concerns neighbors raise
People near a proposed ADU often worry about street parking, loss of privacy from a window facing their yard, noise, trees removed during construction, or a change in the character of the block. Some fear that units will become short-term rentals. Others worry about enforcement, since unpermitted units already exist in many places and may lack safe wiring or exits. Reasonable design standards, such as limits on windows facing neighbors or on height, are a common compromise.
Where ADUs fit among other tools
ADUs are one piece of a broader range of small housing types, sometimes called gentle density. Others include duplexes, townhouses and small apartment buildings on single lots. ADUs are often easier to approve politically because the primary house stays in place and the added home is small, but that same smallness limits their impact. They complement larger efforts toward housing affordability rather than replace them.
For a homeowner considering one, a sensible first step is to read the local code, ask the planning department about size, setback and owner-occupancy rules, and get a realistic estimate of total cost before committing. Rules and costs change, so current local information matters more than any general summary.