Can I Turn the Second Bedroom in My ADU Into an Office or Family Room?

By Buz Artiano, BuildX | October 2026

Watch: We Laid Out a Full-Size ADU Floor Plan in a Warehouse

You're thinking about building a standard ADU plan with a second bedroom, but you don't need a second bedroom. For a household of two, that room is used for storage or it could be an office in place of the kitchen table. Or where the sewing machine can be without getting put away every night. But then you wonder if doing something different like that might become a custom project and cause the price of the ADU to be more than you planned for in the budget.

My name is Buz Artiano, founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We are the only team you need: we do the design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We evaluate septic constraints, zoning issues, utility connections and the requirements for a permit from the city. Massachusetts allows one accessory dwelling unit by right in single family zoning areas, although municipal building code, health code, and dimensional requirements still apply to every project. You can review the state requirements at mass.gov. When a homeowner asks whether a room can change, that question does not get handed off to a subcontractor. We answer it.

We rolled out our 900 foot floor area standard plan at full size on a warehouse floor in Kingston so people could stand in the rooms instead of just looking at a small drawing. Standing on it, the rule was clear: the outside perimeter is set, and the inside is not. That one difference will decide whether your second bedroom will stay a bedroom, and that's worth more to you than any list of options would be.

Can the Second Bedroom Actually Become Something Else?

Yes, with the printed plan in Kingston, the exchange that mattered only took about fifteen seconds. Our host at the Setting the Space outlet walked through the rooms with us and started telling us what people asked for when they don't need a second bedroom: a family room in that area instead, or an office or craft room. Our answer was this: the outside perimeter is set but we can make the inside work for whatever you want.

This idea isn't just us giving in to a hard to please customer. It's how a standard plan is supposed to work. A standard plan is a place to start, and not a final decision. It's there so you have something you can see to react to, and that will change the discussion from "tell us what you want" to "let's move that wall two feet and put the closet on the other side." That second conversation is one homeowners can actually have.

The Two Things on Your Plan That Cannot Move

Two restrictions will keep everything else in place, and both of them deal with the outside of the ADU.

The first is the exterior perimeter: the outside walls, where they will be on your lot, and the shape they make. The second is gross floor area, which on this plan is 900. Gross floor area is measured from the inside face of the outside walls, all the way around. That's the size the plan will have a permit for, and it's the size that every decision on the inside has to work with.

The fixed perimeter comes from this standard plan; the size limit comes from state law. Massachusetts allows an accessory dwelling unit to be built by right at no larger than 900 square feet or half the gross floor area of the principal dwelling, whichever is smaller, and cities and towns can still use their own Title V requirements and their own dimensional rules, including setbacks, which will control how far the building has to be from your property lines. The full requirements are at mass.gov.

One detail confuses people the first time they see the building: the area looks larger than 900 square feet. On our plan there's an outdoor lanai, which is outside the gross floor area measurement but still takes up space on your lot. The perimeter grew but the allowed floor area didn't.

That separation is what gives you room to work. Here's where the limits are when you are talking about rooms.

Everything in the right column is something to talk about, and everything in the left column is a survey, a setback calculation, and a house plan that's been filed. Homeowners who understand that difference can go to the design meeting and ask the right questions, because they won't be trying to negotiate things that aren't negotiable and will spend their time on things that are.

Which Rooms Can I Reassign Inside the Perimeter?

The three trades named were a family room, an office, and a craft room. Those aren't the only three but they are the ones that come up most often when a family of two looks at a plan that's designed for a household of three or four.

The one idea that gets the least attention is the two-person split. Our host talked about it the way a buyer would: it's about how the home will be used, and two people who want one bedroom and one separate private space can have exactly that. A retired couple where one person reads at five in the morning and the other watches television at eleven at night doesn't need two bedrooms, they need one bedroom and one room that has a door.

What actually makes a room swap simple has nothing to do with what the names of the rooms are on the plan. It is what's behind the wall. A bedroom that becomes an office or a family room will keep every wet wall, which means any wall with supply and drain plumbing running inside it, and all of the frame carrying weight down to the foundation, just where the plan originally put them. What will change is the door, the closet, where the outlets are and the sound insulation in the shared wall. Those are drawing decisions. If you want that same room to be another full bathroom, or move it to the opposite side of the plan, then you're relocating supply and waste lines and rerouting the vent stack that carries sewer gas up through the roof. That is a different amount of work and a different cost. We separate those two things on the first day because "it is all customizable" can seem as though every change will cost the same amount and it doesn't.

What Should I Confirm Before I Call the Swap Free?

Deciding what you want to trade is the easy part. Figuring out what it will cost isn't, and so we didn't put a cost to it when we were looking at the plan in Kingston. Four questions have to be answered after "yes, you can change it," and none of them have just one general answer, because the answer depends on your plan and your property. You should get answers to the following four questions in writing before your layout is decided, from us or from any builder you are talking to.

• What does the change do to the price, and is that cost a fee to revise the plan, a construction cost difference, or both?
• What does the change do to the permit set of plans, meaning the drawings your town actually reviews, and does it mean you will have to resubmit plans again if the layout changes after filing?
• What does the change do to the schedule, counting the drawing time and any re-review at the building department?
• What does the change do to anything the local board of health reviews on your specific property?

That last one needs to be explained rather than guessed at. Because towns can apply Title V requirements to ADUs, questions about septic on a specific property need to go to the local board of health. They can't be answered with a builder's rule of thumb and not with a phone call to us. We will direct the call to the board of health before the layout is locked in. A builder who answers that question instantly, either way, without pulling your records, is guessing at your expense.

When Is a Standard Plan the Wrong Place to Start?

You'd think we would tell you that a standard plan works for everyone because a plan we've designed and built before is faster to draw, faster to permit and easier to price. That's our interest in this, and you should be told before you make a decision on anything else we tell you.

It doesn't work for everyone. If a wheelchair-accessible layout is what will determine the width of your doors, your clear floor space and the dimensions of your bathroom, then just renaming the rooms and changing the second bedroom to something else won't help you. If your lot's setbacks force the ADU to have a different shape than the perimeter is drawn for, then the inside isn't your problem. And if you need three separate sleeping spaces inside the 900 foot floor area, you aren't going to get them by reassigning rooms and you need a different plan.

One red flag is worth remembering when you go into any builder meeting. If a builder tells you every interior change is free, that's not generosity. The cost of those changes is being added somewhere you can't see, or it's coming back later as an allowance adjustment, which is the line item a builder adds to when the money for a finish or a fixture turns out not to be enough. Ask which one it is, and ask before you sign.

How to Settle Your Layout Before the Walls Go Up

The decision in front of you isn't as difficult as it looks. You're not choosing between a standard plan and a custom build. You're deciding what will go inside the ADU. The outside is already decided on, and that decision belongs on paper, at design, while a wall is still just a line and to move is only the cost of redrawing it instead of demolition.

Tell us about your household, not the room names. Tell us who's living there, what they do during a normal day, and where they want a door between them. We'll draw the rest.