Hiring guide

How do I know an ADU builder in Maryland is legit?

A legitimate Maryland ADU builder holds a state credential you can look up in two minutes, takes no more than a third down on home improvement work, and gives you the right answer on sprinklers and permits for your county without being prompted. Fail any one of those and keep looking.

This is the check we run before any builder goes on our list. Published November 2, 2026.

Get matched with a builder Start with the license

The credential

Does my ADU builder need an MHIC license or an MHBR number?

It depends on the unit. A detached backyard cottage on a new foundation is new home construction, so the builder registers with the Attorney General and carries an MHBR number. A basement, garage or attached conversion inside the existing house is home improvement, so the contractor holds a Maryland Home Improvement Commission (MHIC) license. Many ADU shops hold both. A builder who cannot tell you which one covers your unit has not built one in Maryland.

Unit	Credential	Issued by	Homeowner protection
Detached backyard cottage, new foundation	Home builder registration (MHBR number)	Attorney General, Home Builder Registration Unit	Home Builder Guaranty Fund, up to $50,000 per claimant
Modular unit set on a new foundation	Home builder registration (MHBR number)	Attorney General, Home Builder Registration Unit	Same fund as a stick built cottage
Basement conversion inside the house	MHIC home improvement license	Maryland Department of Labor, MHIC	MHIC Guaranty Fund, one third deposit cap
Garage conversion or attached addition	MHIC home improvement license	Maryland Department of Labor, MHIC	MHIC Guaranty Fund, one third deposit cap

Look the number up yourself. The MHIC search is on the Department of Labor site and the home builder registration search is on the Attorney General site, both linked at the bottom of this page. Match the legal name on the license to the name on the contract, not the name on the truck. The Home Builder Guaranty Fund pays a homeowner up to $50,000 for losses caused by a registered builder, and the MHIC fund covers home improvement losses the same way, which is the whole reason the number matters.

The 14 Maryland builders we know of, with MHBR numbers where the registry has them

Red flags

What are the red flags when hiring an ADU contractor in Maryland?

The biggest red flag is a deposit above one third, or any money asked for before the contract is signed. Maryland law caps the home improvement deposit at one third of the contract price and bars payment before signing. After that one, the warning signs are mostly about local knowledge.

Says a detached ADU does not need sprinklers. In Maryland it does, in every county, and the county cannot waive it. That line usually comes from California articles.
Says the permit is simple everywhere. Howard, Montgomery, Anne Arundel and Frederick each run a different zoning sign off and a different fee sheet, and a builder who has worked here knows which office stalls.
Cannot name a finished ADU in your county you can drive past.
Quotes a lump sum on a well and septic lot without a health review, a soils test or a sprinkler water supply line in the exclusions.
No written start date and substantial completion date. The state requires both on a home improvement contract.
Insurance certificate is promised for later. A shop that carries general liability and workers compensation can email the certificate in an hour.
The contract names a different company than the license, or lists no license number at all.

Why a detached ADU gets sprinklers in every Maryland county

Problems

Common ADU problems and how to tell them apart

Most stalled Maryland ADU projects fail at one of three desks, zoning, building or health, and each has a different fix. The table sorts the symptom from the cause so you know who to call next.

What you see	Usual cause	Who sorts it
Zoning says no before any drawing is reviewed	The lot or the house fails a threshold: lot size, the footprint cap, owner occupancy, or an HOA covenant the county does not override	County zoning counter first, then a builder
Building permit stalls for months on a detached unit	No sprinkler design in the set. A detached ADU is a new dwelling and gets NFPA 13D in every Maryland county	A sprinkler contractor on the permit set from day one
Health Department holds the permit	Well yield or septic capacity cannot carry a second kitchen and bath, or Montgomery wants a separate reserve area	Soils and septic engineer before the builder prices anything
The garage conversion quote doubles mid job	The slab, the footing or the header will not pass as living space and the job became a tear down and rebuild	A builder who opened the slab and checked the footing before the contract
Builder asks for half down to hold a slot	On home improvement work the state caps the deposit at one third and bars any payment before the contract is signed	Walk away, or pay the capped amount only after signing

The permit checklist, desk by desk

Honest line

Who should not build an ADU in Maryland?

An ADU is the wrong move if you cannot live on the lot, if your budget is under the low end of the published ranges, or if your county has not passed a bill yet. Every county on this site that allows ADUs ties the unit to an owner occupied main house. Frederick bars short term rental of the unit outright. A buyer planning an Airbnb in the backyard of a rental house should stop here.

Baltimore County has no ADU bill yet, and an accessory apartment there must sit inside an owner occupied house. Prince George’s has bills pending. Waiting beats building to a rule that does not exist.
A well and septic lot with no room for a reserve area may never pass the health review no matter who you hire.
The Maryland wide published range starts near $75,000 for a garage conversion and near $120,000 for a detached cottage before sprinklers, septic and utilities. A budget well under that buys a finished basement, not a dwelling unit.
An HOA covenant that bans accessory structures survives the county bill. Read the covenants before you call anyone.

ADU cost in Maryland by unit type

Before you sign

Eight questions to ask any Maryland ADU builder

Ask all eight on the first call. The answers to the first three settle most of the field.

Which number do you hold for this unit, MHIC or MHBR, and can I look it up tonight?
How many ADUs have you finished in this county, and can I drive past two of them?
Who designs and installs the NFPA 13D sprinkler system on a detached unit, and is it in your price or outside it?
Who pulls the zoning sign off, the building permit and the fire protection permit, and who pays the fees?
What is excluded: septic work, the utility tap, the driveway, the sprinkler water supply, the landscaping?
What deposit do you take, and what are the payment milestones after it?
What are the approximate start and substantial completion dates, in writing?
Send me the certificate of insurance today, general liability and workers compensation, with my address as the certificate holder.

Every lot is different. What a unit will cost on your lot, in your county, with your septic and water situation is a call, not a page.

Get matched

Start with a builder who passed this check

Send the address and the kind of unit. We confirm the county rule and the sprinkler answer, then connect you with a licensed Maryland builder who already does ADUs there. We do not build, and we do not rank anyone.