Adding a backyard apartment or in-law suite to a new custom home: the local rules

By Builder Introductions Team · Updated Nov 10, 2026 · How we build this directory

A lot of families building a new house want room for parents, grown children or a caregiver, with its own kitchen and entrance. Building that space in from the start is far cheaper than adding it later, but each county treats these units differently, and the rules are changing in 2026 and 2027.

What counts as an accessory unit

An accessory dwelling unit (ADU) is a second, complete home on the same lot as a single-family house, with its own kitchen and bathroom. It can be inside the main house (often the basement), attached as an addition, or detached in the backyard. Fairfax County calls them accessory living units.

Montgomery County

Montgomery County allows accessory dwelling units, attached or detached, and building one is mostly a by-right process handled by the Department of Housing and Community Affairs and the Department of Permitting Services (Montgomery Planning staff report). One ADU is allowed per lot. A new state law requires every Maryland county to have a local ADU law consistent with it by October 1, 2026, and the county has been amending its rules to match (ZTA 26-06), including proposed changes to the maximum size. Short-term rental of an ADU is not allowed in Montgomery County.

Arlington County

Arlington allows accessory dwellings inside or attached to a one-family house, and in detached accessory buildings, and it has loosened its rules for detached units more than once. Check the county’s accessory dwelling page for the current size, occupancy and parking rules before you design.

Fairfax County

Fairfax County allows interior and attached accessory living units by administrative permit. Detached units have been much harder to get, and in 2026 the county began reviewing its rules after relatively few were approved, with staff proposing to allow detached units more easily on larger lots. Check the current status with the county before you design one.

A new Virginia law in 2027

Virginia passed a statewide law that requires localities to allow accessory dwelling units, taking effect in 2027. How each county applies it, and whether your HOA covenants still restrict it, will matter. If you are designing now for a build that finishes in 2027 or later, ask your architect to design so a unit can be finished later.

Designing it in from the start

Basement units are the cheapest: a walk-out or egress windows, a separate entrance, a kitchenette and a bathroom.
Plan the utilities. A separate electrical panel or subpanel, sound insulation in the ceiling, and a separate HVAC zone make the unit livable.
Mind the coverage limits. In Arlington and Montgomery County, a detached unit counts against lot coverage, which may shrink the main house.
Fire separation. Building codes require fire-rated separation between the two units. Your builder will know the details.

Use the lot checker to see your zoning and coverage limit, and ask builders on your shortlist whether they have built one.

Frequently asked questions

Can I build a detached backyard unit in Montgomery County?

Generally yes. Montgomery County allows one attached or detached ADU per lot, mostly by right, subject to its size, setback and licensing rules.

Does Fairfax County allow detached accessory units?

Interior and attached units are allowed by administrative permit. Detached units have been much harder, and the county is reviewing its rules in 2026, so check the current status.

Is a basement apartment the cheapest option?

Usually. In a new house, a basement unit mostly needs a separate entrance, a kitchenette, a bathroom and fire separation.