ADU Builder in Maryland — Rules, Permits & Costs

Last updated August 2026. County ordinances are actively changing under the Maryland ADU Act — we verify the current rules for your specific address as the first step of every project.

Accessory Dwelling Units in Maryland: The Complete 2026 Guide

Maryland just became one of the most ADU-friendly states on the East Coast. The Maryland ADU Act of 2025 requires every county and municipality to allow accessory dwelling units on single-family lots by October 1, 2026 — which means basement apartments, in-law suites, and backyard cottages are becoming buildable in places they never were.

Karma Construction Group designs, engineers, permits, and builds ADUs across Montgomery County, Prince George’s County, Howard County, and Frederick County under one contract — including the licensing steps that trip up most Maryland homeowners.

The Maryland ADU Act (HB 1466) — What It Requires

Signed April 2025 (passed the House 138–1 and the Senate 47–0), effective October 1, 2025:

• Every jurisdiction with zoning authority must adopt a local law authorizing ADUs on single-family detached lots by October 1, 2026
• An ADU may be attached or detached, up to 75% of the primary dwelling’s size
• ADUs are excluded from density calculations and residential growth limits
• ADU setbacks can’t exceed existing accessory-structure side/rear setbacks
• New parking mandates are restricted — allowed only after a parking study, with a waiver process
• HOAs and deed covenants may not unreasonably prevent you from building or renting an ADU (short-term-rental limits are still allowed)

Owner-occupancy rules remain a local decision — and most Maryland counties do require them, as detailed below.

ADU Rules by County (as of 2026)

County	ADUs Allowed?	Max Size	Owner-Occupancy	Key Extra Step
Montgomery	Yes (since 2019)	1,200 sq ft attached / least of 50% footprint, 10% lot, 1,200 sq ft detached	Yes	DHCA Class 3 ADU license ($615)
Prince George’s	Ordinance pending — due Oct 1, 2026	Will follow state 75% cap	TBD	Fire sprinklers required in new builds
Howard	Yes — by right (Apr 2026)	75% of principal footprint	Yes	DPZ permit + DILP building permit
Frederick	Yes	≤1,000 sq ft by permit alone; larger by special exception	Yes	Impact fee waived ≤800 sq ft

Montgomery County ADUs

Montgomery has been Maryland’s ADU leader since ZTA 19-01 took effect at the end of 2019, allowing attached and detached ADUs without a special-exception hearing on single-family detached lots (R-200, R-90, R-60 zones for detached units).

Size and use rules: attached ADUs up to 1,200 sq ft (a basement ADU may use the full basement); detached ADUs capped at the least of 50% of the main home’s footprint, 10% of the lot, or 1,200 sq ft. One ADU per lot, owner-occupancy required, occupancy capped at 2 adults plus children, and no short-term rentals. Parking: one on-site space — waived entirely within 1 mile of any Metrorail, Purple Line, or MARC station, which covers a large share of the county.

The step most people miss: every Montgomery ADU needs a DHCA Class 3 ADU rental-housing license — even if you never rent it. The application currently totals $615 ($490 in Takoma Park), includes a 30-day public-notice yard sign and DHCA inspections, and licensing typically runs 2–4 months alongside your DPS building permits. Construction can’t begin before DHCA’s preliminary inspection report — sequencing this correctly is exactly the kind of thing our permitting team manages so your project doesn’t stall.

Prince George’s County — ADUs Arriving by October 2026

Prince George’s currently does not permit ADUs — the county removed ADU provisions from its zoning rewrite, and existing units are unpermitted. But under the state Act the county must authorize ADUs by October 1, 2026; an ADU Task Force has finished its work and draft legislation is in the final adoption stages.

Two things PG homeowners should know now: the coming rules must conform to the state’s 75%-of-primary cap and setback limits, and Prince George’s requires automatic fire sprinklers in all new residential construction — including detached ADUs (roughly $1–$2/sq ft in new construction). If you design now, you file the day the ordinance takes effect — and you’re first in a very long line.

Howard County ADUs — By Right Since April 2026

Howard County’s CB 3-2026 (effective April 8, 2026) made ADUs by right in residential districts, aligned with the state Act: interior, attached, or detached; up to 75% of the principal dwelling’s footprint; maximum 2 bedrooms; owner-occupancy required; no short-term rentals. Lot-size minimums were removed, ADUs are exempt from school-adequacy (APFO) tests, and Columbia’s New Town district is included subject to its development-plan standards. You’ll need a Department of Planning and Zoning permit plus a building permit — and a rental license if you rent to non-family. Well/septic lots need Health Department sign-off, which we coordinate during design.

Frederick County ADUs

Frederick County (unincorporated areas) splits ADUs by size: units of 1,000 sq ft or less are a permitted accessory use needing only a building permit — no hearings. Larger units require a Board of Appeals special exception. All ADUs are capped at half the size and half the footprint of the main dwelling, owner-occupancy is required, one parking space is needed (street parking can count), and short-term rentals are prohibited. A meaningful cost break: ADUs up to 800 sq ft are exempt from the county’s school impact fee. The City of Frederick and other municipalities set their own rules — we confirm your exact jurisdiction first.

What an ADU Costs in Maryland

Planning ranges for suburban Maryland (from the Prince George’s Planning Department’s ADU study and Maryland builder-published 2026 figures):

ADU Type	Typical Range	Notes
Basement conversion	$80,000 – $200,000	Least site work; egress + kitchen drive cost
Garage conversion	$75,000 – $150,000	Insulation, HVAC, plumbing upgrades typical
Attached addition ADU	$100,000 – $250,000	Blends with the home like a standard addition
Detached backyard ADU	$120,000 – $300,000+	~$180–$400/sq ft in the DC-suburb market
Modular/prefab ADU	$150,000 – $200,000	Faster, more cost-predictable

Design, engineering, and permitting typically add $8,000–$25,000 depending on county and complexity; utility connections commonly run $5,000–$15,000 for detached units; and in Prince George’s, sprinkler systems add roughly $650–$1,300 for a typical new 650 sq ft unit.

How We Build Your ADU — One Contract, Four Phases

1. Feasibility & design — zoning check for your exact address, utility assessment, and a design matched to your county’s size caps and your budget.
2. Engineering, permits & licensing — structural drawings, building permits, and the county-specific extras: Montgomery’s DHCA license and public-notice process, Howard’s DPZ permit, Frederick’s impact-fee positioning. Permitting is a core competency.
3. Construction — one project manager, our own crews, a published schedule.
4. Inspections, license & occupancy — final inspections and every certificate needed to legally occupy (or rent) the unit.

ADUs pair naturally with a home addition or a whole-home remodel — and if you’re in Virginia, see our companion guide to ADU rules and costs in Northern Virginia.

Why Maryland Homeowners Are Building ADUs Now

• The law is on your side — the state Act even limits HOA interference
• Rental income in the DC suburbs’ deep rental market
• Multi-generational living — parents or adult kids, close but independent
• First-mover advantage — PG County opens October 2026; designed projects file first

Ready to find out what’s possible on your lot? Call 202.798.7663 or request a free ADU feasibility consultation below — we’ll give you a straight answer on zoning, licensing, timeline, and budget.