ADU rules in Howard County
Howard County passed its ADU bill in February 2026 and it took effect on April 8, 2026. A homeowner in Columbia, Ellicott City, Clarksville or Elkridge can now add a second, smaller home on the lot: attached to the house, detached in the yard, or carved out inside the house.
This page is the short version of the rule, the permits you will meet in the order you meet them, and what the published numbers say an ADU costs. When you are ready, the form at the bottom gets you a licensed Maryland builder who already works in Howard County. We do not build the units. We make the match.
CB 3-2026 (ZRA 218)
In force since April 8, 2026
Council Bill 3-2026, filed as Zoning Regulation Amendment 218, was signed on February 6, 2026 and became effective on April 8, 2026.
It allows an accessory dwelling unit attached to the house, detached from it, or inside it, on a single family lot.
The county webinar on ZRA 218 (answer 18) says the bill does not change the building code, so the sprinkler question is answered by the building code, not the zoning bill.
Read on the county planning and zoning ADU page and the ZRA 218 webinar questions and answers, October 31, 2026.
What Howard allows
- Size: up to 75 percent of the principal dwelling footprint.
- Bedrooms: two at most.
- Occupancy: the owner lives on the property, in the main house or the ADU.
- Placement: attached or detached, and a unit inside the existing house also counts.
- The state law behind it (Chapter 196 of 2025) also sets 75 percent of the main house as the ceiling, so Howard matches the state cap.
Detached, attached or basement
Detached
A backyard cottage is a new one family dwelling in the eyes of the Maryland building code. It needs its own foundation, its own utility connections or taps, and a full NFPA 13D fire sprinkler system. It is the most private of the three and the most expensive.
Attached
An addition with its own entrance and kitchen. It shares a wall, often a roof, and usually the water and sewer line with the house. Under the state code an addition to a house that has no sprinklers does not trigger sprinklers on its own, unless the addition is large: Howard treats an addition at 50 percent of the gross floor area as the trigger.
Basement
The cheapest route when the basement already has height and a way out. Egress, a second exit, moisture and ceiling height are what decide it. Inside an unsprinklered house it is an alteration, and the state code does not force sprinklers for it.
Permits in Howard, in the order you meet them
- 1
Zoning
The ADU has to fit the lot under ZRA 218: size against the main house footprint, two bedrooms, owner occupancy. Planning and Zoning signs off before the building permit moves.
- 2
Building permit
A residential building permit from the Department of Inspections, Licenses and Permits. A detached unit files as new residential construction; a conversion or addition files as an alteration or addition.
- 3
Fire protection permit
A detached ADU gets a 13D sprinkler design and its own sprinkler permit. An attached or basement unit in an unsprinklered house normally does not.
- 4
Health (well and septic lots)
On a well or septic lot the Health Department has to confirm the well yield and the septic capacity carry a second kitchen and bath before the building permit issues. Public water and sewer lots skip this step.
- 5
Utilities
Public water and sewer: a tap off the house line or a new service, sized for the sprinkler flow on a detached unit. Electric: a sub panel or a second meter, your call with the builder and BGE.
The whole list, statewide, in the order a homeowner meets it:
ADU permit checklistWhat an ADU costs in Howard County
No builder publishes a Howard County specific ADU price. The ranges below are Maryland wide, from one named builder page. Ask the builder for a Howard number off your lot and plans before you budget.
| Type | Published range |
|---|---|
| Detached new build | $120,000 to $300,000 and up |
| Attached addition | $100,000 to $250,000 |
| Basement conversion | $80,000 to $200,000 |
| Garage conversion | $75,000 to $150,000 |
Source for the table:
Clarksville Construction Services, "How much does it cost to build an ADU in Maryland"County fees
County permit fees on top of construction, from the FY2027 Building Permit Fee Schedule (effective July 1, 2026):
- New residential construction filing fee: $0.25 per new gross square foot.
- Additions to residential construction: $50 filing fee.
- Alterations and repairs to a single family dwelling: $25 filing fee, then $50 for a single room or area up to 200 square feet or $100 over 200 square feet.
- A 10 percent technology fee on every permit. Excise tax and the school facilities surcharge are separate schedules on the same county page.
Source for the fees:
Howard County FY2027 Building Permit Fee ScheduleSprinkler cost
A detached unit adds the NFPA 13D sprinkler system. On public water that runs about $1.35 to $1.60 per sprinklered square foot (Maryland Fire Sprinklers cost guide, marylandfiresprinklers.com/cost/). A well lot adds a storage tank and pump, priced on the same guide.
Statewide ranges and how the types compare:
ADU cost in MarylandThe sprinkler question, answered once
Every detached ADU in Howard County gets an NFPA 13D sprinkler system. The county cannot waive it (Public Safety Article 12-504) and the ZRA 218 webinar says the zoning bill leaves the building code alone. Our sister company, Maryland Fire Sprinklers, designs, permits and installs them.
A licensed builder for your Howard lot
Send the address and the kind of unit. We check it against the Howard rule and the sprinkler question, then connect you with a licensed Maryland builder who already does ADUs here.