Construction

There are two types of construction to consider and both have pros and cons, so think about the level of customization you want and how the ADU will work on your property.

Site-built ADUs
Traditionally constructed and built from scratch on your property (AKA “stick-built”), which often slows for a lot of customization and smaller changes throughout construction.

Typical benefits:

• More options for customization
• Can be easier to get financing and permits
• Can be easier to place on unique properties (steep slopes, rocky terrain, small spaces)
• Can typically be paid for phase-by-phase

Prefab, modular, and panelized ADUs
Built in a factory, then shipped to your site and assembled or placed on a foundation. Some companies provide “turnkey” service with support through the entire process (permitting, construction, etc.). If not, you’ll need to hire other professionals to help.

Prefab ADUs are categorized based on how they are built and inspected.

• Partially fabricated (panelized, framing kits) Parts of the ADU are built off site at a factory, and construction follows a traditional site-built pattern. Local inspectors check work at different stages and a local general contractor and their crew are responsible for putting all the pieces together and passing inspections.
• Prefabricated (modular, manufactured, factory-built) The ADU is built and inspected for building code off site at a factory, then delivered and put in place on your property by a crane or tow. Local inspectors only check work like foundation and utility hook-ups. These types are all built similarly but are called different things based on what building code they meet (state, federal, etc.).

It’s important to note that a prefab ADU project may cost the same as site-built ADUs when you factor in transportation, crane, foundation, and sales tax, which can end up being 20-40% of the total cost. When you get in touch with prefab companies, be sure to ask what is included in the price. Some companies do offer a lower price point.

Typical benefits:

• Lower design and engineering costs
• More clarity on what you’ll get in the end
• Faster construction with less on-site disruption
• Quality inspections from the start
• Fewer permits and inspections

If you are building a site-built ADU from scratch on your property and you’re not using a design/build firm, you will need to find a contractor to take over for the construction phase of your ADU.

First, you’ll solicit bids – ideally at least three for comparison. Then you can begin selecting your contractor. Before you hire a contractor, make sure to check their license and insurance and when they present you with a contract, review everything carefully.

The state runs the Home Improvement Contractor (HIC) program with many more resources on finding and hiring a contractor. You can also see our Pioneer Valley ADU Accelerator page for a list of modular and panelized designs and companies that work in the Pioneer Valley region of the state and beyond.

Traditional stick-built construction will take 6-10 months, though this will vary depending on the specifics of the project. Stages of stick-built construction include:

• Site preparation: 1-2 months
• Foundation: 1 month
• Walls, roof, doors: 1-2 months
• Plumbing & electrical: 1-2 months
• Insulation & drywall: ½-1 month
• Fixtures & finishes: 1-2 months
• Final touches: ½-1 month

Modular construction is typically quicker than traditional construction. A factory typically needs a few months to order all the materials and components, then takes about a month to assemble them while the site and foundation is being prepared. The modular “boxes” are often delivered and assembled within a day or two, and then the local contractor can take 1-3 months to finish any remaining items, including siding, custom items, landscaping, drywall connecting the boxes and painting.

Check with your city or town’s building department to confirm timeline for your specific project.