Picking an ADU house plan feels like the fun part. And it is, but only after you have answered a few questions that most homeowners skip. The right plan for your property is not the one that looks best online. It is the one that fits your lot, your budget, and your town's rules. Here is how to think about it.
Why your lot comes before your plan
A lot of homeowners start by browsing floor plans first. That always felt backwards to us.
Remember: your lot dictates almost everything. How much square footage you can build, where the structure can sit, how the entrance needs to be oriented, and what utility connections are possible. A 900 square foot plan that does not fit within your setbacks is not a plan. It is a problem waiting to happen.
Before you fall in love with a layout, confirm what your property can actually support. That means knowing your setbacks, lot dimensions, and septic situations. Design should follow feasibility, not the other way around.
What ADU plans need to include in Massachusetts
Under the Massachusetts Affordable Homes Act, any ADU plan must account for the following:
- A separate entrance, either directly from outside or through a shared corridor that meets state building code egress requirements
- A maximum size of 900 square feet or half the gross floor area of the primary home, whichever is smaller
- Compliance with local setback and dimensional requirements set by your town
- Two separate means of egress per Massachusetts building code
- A one-hour fire separation between an attached ADU and the existing dwelling
- Smoke and CO detectors interconnected with the primary home where applicable
Plans also need to be stamped by a licensed architect before a building permit can be issued. That is a requirement regardless of where the plans come from.
Layout options and what they are good for
Most ADU house plans fall into a few standard configurations. What works best depends on who is living there and how the unit will be used.
Studio layout
(one open space with a kitchen, bath, and sleeping area): Best for a single occupant, a caregiver, or a rental unit where lower rent is the goal. Most affordable to build per square foot.
One-bedroom layout
The most common ADU configuration. Works well for a family member, a long-term renter, or a couple. Enough separation between living and sleeping to feel like a real home.
Two-bedroom layout
Closer to the 900 square foot maximum. Better suited for a small family or two roommates. More build cost, but more rental income potential too.
For any layout, prioritize natural light, practical storage, and an entrance that feels intentional, not like an afterthought bolted onto the side of the main house.
Free plans vs. custom design
Massachusetts recently launched an ADU Design Challenge that produced free, downloadable ADU plans from architects and designers across the state. These are a legitimate starting point, especially for detached builds on straightforward lots.
The limitation is that off-the-shelf plans are designed for ideal conditions. If your lot has irregular dimensions, a tight footprint, challenging terrain, or specific setback constraints, a generic plan may not get you through permitting without modifications.
Custom design costs more upfront but produces plans built around your actual property. For most Massachusetts homeowners, particularly those with non-standard lots, custom is the more reliable path.
Who draws the plans and who builds to them
This is the question most people ask last. It should be one of the first.
If your designer and your builder are separate, there is a gap between the plan and the build. Things change during construction. Decisions get made on the fly. The vision you approved can drift.
A design-build firm draws the plans and builds them. One team is accountable for both. What you sign off on is what gets constructed.
At BSDC, design and construction are handled together. We produce a full 3D render so you can see exactly what you are approving before we break ground, and we build to those plans ourselves.
If you live in Massachusetts and want a plan that makes sense for your property, give us a call. We are happy to answer any questions!
What's possible on your property?
Tell us about your project and we'll schedule your free consultation.