ADU Construction Services Guide: From Design to Final Build

Not all ADU construction services are the same. Some firms handle design. Others only build. A few do both. What you hire determines how smooth your project goes. This post breaks down what a full-service ADU build actually looks like and what to expect at each stage.

Design-build vs. design-then-build

Most homeowners do not realize there are two very different ways a project can be structured.

Option 1 = Design-then-build 

This is when you hire an architect separately, get plans drawn, then hire a contractor to build to those plans. Two separate teams, two separate contracts, two separate points of contact. When something gets lost between the drawing and the build, and something almost always does, you are in the middle trying to figure out who is responsible.

Option 2 = Design-build

Or you can hire one firm to handle everything. Design, permits, and construction under one roof. One team owns the project from the first conversation to the final walkthrough. What you approve is what gets built.

For an ADU, which involves zoning compliance, utility work, permitting, and detailed construction, the design-build model is a significantly lower-risk path. There is no handoff between teams where your project can get lost.

What a full-service ADU process looks like

Every firm runs their process a little differently, but a thorough ADU build follows these phases:

1. Feasibility first. Before any design work starts, a good builder assesses your lot. Setbacks, zoning, utility capacity, septic situation, realistic budget. This step exists to tell you what is actually buildable on your property before you spend money designing something that does not work.
2. Design and 3D rendering. Once feasibility clears, design begins. You work through layout, finishes, and how the unit functions. A good design-build firm will produce 3D renders to see exactly what you’re approving before construction documents are finalized.
3. Permits and approvals. Your builder handles permit submissions to your town's building department. Under Massachusetts' Affordable Homes Act, compliant ADUs are approved by right, meaning no special permit or public hearing. But permit timelines still vary by town, and having an experienced builder who knows your local process matters.
4. Construction. Once permits are in hand, the build begins. Foundation, framing, mechanical systems, insulation, finishes. A design-build firm manages all trades and keeps you updated without you having to chase anyone down.
5. Final inspection and walkthrough. Construction wraps with inspections and a final walkthrough to make sure everything is right before you take the keys.

How long does it take?

Realistically, from first conversation to move-in, you should plan for six to twelve months. Here is how that breaks down:

• Feasibility and design: one to three months
• Permitting: one to three months, depending on the town
• Construction: two to four months for an interior conversion, four to six months or more for a detached new build

Permitting is usually where projects slow down. Towns with clear ADU bylaws move faster. Others take longer. Working with a builder who has already navigated your town's process cuts that timeline down.

What to look for in an ADU construction service

A few things worth confirming before you sign with anyone:

1. Do they handle design and construction together? If not, you carry the coordination risk between two separate teams.
2. Do they run a feasibility check before design? Anyone who skips this step is selling you a design before they know if your lot can support it.
3. Have they built ADUs in Massachusetts specifically? State and local rules matter. You want a team that has pulled ADU permits in your area, not one learning your town's process on your project.
4. Is there one point of contact? You should not have to track down multiple people to get an answer about your own project.

At BSDC, our ADU process starts with a paid feasibility and budget roadmap ($3,500, credited in full if you move to construction), so you know exactly what your property can support before committing to a full build. From there, we handle design, permits, and construction with one team and one point of contact, start to finish.

If you’re a Massachusetts homeover and want to talk through what your project would look like, give us a call.