Match the Type to the Property
Four configurations, four very different design problems
There’s no single “best” ADU. A standalone cottage, a garage conversion, a basement conversion, and an attic conversion each solve the problem differently — and each carries its own technical hurdles. Knowing what drives cost and feasibility in each one is how you avoid an expensive surprise halfway in. Here are the considerations we weigh for every type.
A new, freestanding building
The most flexible option — and the one most shaped by your land and septic system.
- Siting & setbacks — where it can legally and sensibly sit on the lot
- Septic & Title 5 — capacity and bedroom counts, a universal Outer Cape constraint
- Foundation & drainage — soil, grade, and water table on your specific parcel
- Utility runs — distance from the main house drives real cost
- Scale & character — a building that belongs beside the main house
Reusing structure you already have
Often the most cost-effective path — if the existing structure cooperates.
- Foundation & slab — whether the existing base is adequate for living space
- Insulation & weatherization — bringing a cold structure up to a comfortable, code-compliant standard
- Ceiling height — meeting habitable-room minimums under the existing roof
- Egress & light — adding windows and exits for a legal dwelling
- Parking trade-offs — what you give up and how the site adapts
Finishing the space beneath your home
Hidden potential — governed by egress, ceiling height, and moisture.
- Egress windows — legal escape and natural light are non-negotiable for a bedroom
- Ceiling height — headroom below beams and ductwork makes or breaks feasibility
- Moisture & flood zone — waterproofing and elevation, critical near the coast
- Radon & air quality — testing and mitigation designed in
- Separate access — an entrance that gives the tenant true independence
Building up, into the roof
Character and views — constrained by headroom, access, and structure.
- Headroom — usable floor area under a sloped roof, often the deciding factor
- Stair access — a compliant, comfortable stair without sacrificing the floor below
- Structural load — whether the existing framing can carry a living space
- Dormers & light — adding volume, headroom, and egress where it counts
- Insulation & climate — keeping a top-floor unit comfortable year-round
One More Option
Modular & prefab
For some properties, a modular or prefab unit offers speed and predictability. The
design considerations — siting, septic, foundation, and utilities — still
apply, and we help you weigh whether it fits your lot and goals.
Explore modular & prefab →
A disclaimer worth stating plainly
Every consideration above is general guidance, not a determination for your property. Septic capacity, zoning, historic-district review, and flood-zone rules vary by town and by parcel. We confirm the specifics with your local building department as part of the design process — and we always encourage you to do the same.
Let’s Talk
Good design starts with a conversation.
Tell us about your home and what you’re trying to accomplish. The first consultation is free, and you’ll have a preliminary design in hand before any estimate — no obligation, no pressure.
Start with a Free ConsultationVern Hatt · Licensed Construction Supervisor #CS62739 · Home Improvement Contractor #177590