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Detached, Attached, or Converted: Which ADU?

The three ADU paths head-to-head — cost shape, timeline shape, privacy, and who each one fits. The decision page for shoppers not yet committed.

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Free on-site estimates  ·  No-pressure advice  ·  Waldorf and Charles County, MD

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Backyard cottage ADU behind a suburban Maryland home

The three types in one glance

Detached: a separate small home on your lot — a backyard cottage. Most expensive (Makara ADU publishes $150,000–$350,000), most private, most design freedom. Needs full zoning treatment: setbacks, height caps, utility trenching.
Attached: an addition-style ADU sharing a wall with your house — see attached ADUs. Mid-range cost, shares systems (shorter utility runs), less private, must blend with the main house.
Garage conversion: your existing garage, transformed — see garage conversion ADUs. Least expensive (Foster Remodeling publishes $50,000–$200,000 for conversions), fastest to build, but you inherit the garage's bones, ceiling height, and location.

Sources: Makara ADU (quoted in The Banner); Foster Remodeling (fosterremodeling.com, published figures).

Cost shape

Detached costs the most because everything is new from the foundation up. Attached saves on one wall, shared systems, and shorter utility runs — but still needs foundation work and a roof tie-in. Conversions skip structure entirely and pay only for transformation. The gap closes when the existing structure fights back: a garage with bad bones can approach attached-ADU money for a compromised result.

A note on prices: material costs move up and down, and no two lots need the same work. Treat any numbers on this page as starting points, not quotes. Your written estimate after our free lot walk is the real number for your project.

Privacy and daily life

This is the factor spreadsheets miss. A detached ADU is a separate home — own entrance, no shared walls, tenants or parents live independently. An attached ADU shares a wall (and sometimes a laundry or hallway decision). A conversion's privacy depends on the garage's placement — detached garages convert into surprisingly private units. If the ADU is for aging parents, togetherness may be the point; if it is for rental income, separation is what tenants pay for.

Zoning notes per type

Charles County's published draft treats the types slightly differently: attached ADUs meet the main house's setbacks; detached units on smaller lots need 6-foot side/rear setbacks and must sit 6 feet behind the home's front building line; one ADU per lot total — never both an attached and a detached unit. Attached-ADU entries and stairs must not be visible from the street. Conversions must still meet ADU standards — a garage does not become an ADU by declaration.

Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft, §§3.01.700.E, G, I).

The decision

Choose detached for maximum privacy, rental income, or design freedom — and the budget to match.
Choose attached for aging parents who want closeness, or when the lot has no room for a separate building.
Choose conversion when the garage has good bones and the budget is the boss.
Still torn? Our detached vs. conversion cost comparison puts numbers on two of the three — or book the free feasibility study and decide with your lot in front of you.

Questions homeowners ask us

Which ADU type is cheapest?

Garage conversions, by a wide margin — the structure already exists. Foster Remodeling publishes $50,000 to $200,000 for conversion-type projects. Attached ADUs sit in the middle; detached new builds cost the most. But a garage with bad bones can erase the savings, so price your actual structure.

Which ADU type is best for rental income?

Detached, usually — tenants pay for privacy, own entrances, and no shared walls. A detached-garage conversion is the budget version of the same advantage. Attached units rent fine but command less separation premium.

Which ADU type is best for aging parents?

It depends on the family. Attached ADUs keep parents close with easy check-ins; detached cottages give everyone independence with proximity. Either way, prioritize no-step entries, walk-in showers, and one-level living over square footage.

Can I build both an attached and a detached ADU?

Not in Charles County under the published draft rules — one ADU per lot, and a lot may not have both types. One per lot is also the industry-typical pattern across Maryland counties.

Does the ADU type change the permit process?

The sequence is the same — health-department sign-off first where it applies, then the ADU permit, then the building permit. What changes are the details reviewed: setbacks differ for attached vs. detached, and conversions are reviewed against ADU dwelling standards.

What happens after you reach out

No mystery, no sales script. Here is the whole path from first call to move-in day.

1You reach outCall or send the form. Tell us about your lot and what you want the space for — a parent, rental income, a home office.
2We walk your propertyFree visit. We check setbacks, access, slope, trees, and utilities, and tell you honestly what fits.
3You get a plan and a priceA site plan and a written estimate for your lot. No pressure, no obligation, no fine print.
4Permits, then buildWe handle the Charles County paperwork and follow-ups. Then our partner builders build it.

Not sure what fits your lot? Ask us.

One call settles it — we will tell you what your property can hold and what it would take.

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