Why Does an ADU Floor Plan Shrink Once the Walls and Hallways Are Drawn?

By Buz Artiano, BuildX | October 2026

Watch: Designing Concepts & Tools for Net Zero Homes w/ Gwendolen St. Sauveur | BuildX Podcast #18

You've drawn the floor plan for your new ADU on some graph paper. You've got a kitchen, two bedrooms, a bathroom, and a small laundry area, and it all fits just fine in the square footage that you can have. But when you had a professional do the same floorplan with the walls and everything included, the rooms shrunk. Yet they're not smaller than the ones you drew and did the math on yourself. This can be extremely frustrating, especially after you have already made plans for what those rooms will be used for. There's a good reason for the difference between the two drawings, and once you understand it, you can plan around it from the beginning and avoid this frustration.

My name is Buz Artiano, and I am the 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. And when the sketch is turned into an actual drawn plan it happens with us. 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 requirement to get a permit from the city.

On BuildX Podcast #18, Gwendolen St. Sauveur of BTF Net Zero Designs talked about the sketches homeowners send her, where every wall is just a pencil line. Our answer on camera is just this: it will all fit when the wall is only an eighth of an inch wide. An actual built wall has a thickness and rooms have to have hallways that connect them. Both of those things will take up floor space long before countertops are installed, and that loss of space is easier to deal with when you know its size.

Why Did My Floor Plan Shrink When the Walls Were Drawn?

Because a wall on paper doesn't actually have any width, and a wall in a building does. An inside wall is framed with two-by-four studs, which measure 3.5 inches, and finished with half-inch drywall on each side. That adds up to 4.5 inches of floor for every interior wall. So each interior wall on the floor plan will take up a strip of floor that the sketch counted as part of a room.

Circulation is the second cause. Circulation means the floor that you walk across to get to a room such as an entry, a hallway to the bedrooms, or the empty space in front of a closet. On paper, rooms share a line and touch each other. But in a house, each room has to have a path to the next one, and that path takes up floor area that isn't part of any room at all.

Gwendolen sees this in the sketches that clients send BTF. Her first response back to them is that the drawing is missing about 15 percent, which is the same 14 to 16 percent allowance for walls and hallways we will discuss below, so some floor space is going to be lost. What the drawn plan really measures is the ADU before it's actually framed. It measures rooms that have no walls between them.

How Much Square Footage Do Walls and Hallways Take?

The only measured numbers we've seen for this issue come from BTF Net Zero Designs. In the firm's library of plans 14 to 16 percent of finished square footage is either hallway or the thickness of the walls. All of their designs are under 2,000 square feet, and most of them are between 1,300 and 1,500 square feet. They are houses and we haven't come up with an equivalent percentage for BuildX ADU plans, so just use this range as an estimate from a designer who counted, and then get a drawn plan for your ADU to replace it. BTF didn't say how that finished square footage was measured, so their figures might not be exactly the same as the figures Massachusetts uses to measure an ADU.

The table below uses that benchmark arithmetic for four different sizes. It doesn't represent a particular floor plan or make a promise about any project. It just shows what the ratio would mean if it is correct, which will give you a working number until you have a real plan for your property.

The bottom row matches the size range that the benchmark was measured on. A 1,300-square-foot house still has about 1,100 square feet left over for rooms. At 600 square feet, about 510 square feet remains, and the same percent will be taken out of an ADU that still needs a kitchen, a bathroom, and a bedroom.

Why Does This Matter More in an ADU Than in a House?

Because the ADU has a ceiling that's fixed. Massachusetts allows one ADU by right on lots zoned for single-family homes, and that means the ADU is allowed under state law without getting a special permit for it. The size of the unit is limited to 900 square feet or half the gross floor area of the principal dwelling, whichever is smaller. Local building code, health code, and dimensional requirements still apply. You can review the current requirements at mass.gov.

How that gross floor area is measured is important. The state ADU regulations count it from the interior side of exterior walls, include only the floors of those rooms whose ceiling height complies with the building code, and eliminate crawl spaces, garage parking areas, attics, and enclosed porches. The complete definitions are in the regulations at mass.gov. Exterior wall thickness inside included in that measurement so it never comes out of your 900 square feet. But inside walls and hallways are what will reduce the floor space left for rooms.

In a house, the benchmark is a question of efficiency. A 1,500-square-foot plan that loses 15 percent still has about 1,275 square feet for rooms, and with no size limit from the state, the owner could choose a bigger ADU. An ADU that has a size limitation can't be built any larger. So if the inside walls and hallways took 15 percent of a 900-square-foot ADU, about 765 square feet would remain for rooms, and that space still has to include a kitchen, bathroom, bedrooms, laundry, and the mechanical equipment. Think of that number as an illustration. The actual share depends on your layout, and remember that the BTF range was measured on houses.

Where Does the Lost Space Go in a Small Plan?

The thickness of the walls uses floor space at about the same rate everywhere there's a wall. The circulation part behaves differently. It will be wherever the plan needs a path between rooms, and in a small unit the longest path is usually the hallway to a bedroom at the back. A hallway 3 feet wide and 12 feet long uses 36 square feet. In a 900-square-foot ADU, just that single hallway is 4 percent of the whole ADU.

For the builder, a hallway floor gets framed, insulated, drywalled, wired, lit, and heated in the same way a bedroom does, so it costs about what a bedroom floor costs per square foot. A reader comparing two different layouts with the same square footage should compare how much of each one is a path and how much is a room. That comparison should be made with the first draft, while shortening a hallway is just moving lines of paper.

Gwendolen calls her small plans tiny, big homes. At 1,500 square feet they still have a full kitchen, lots of living space, and sometimes a walk-in closet. Her layouts were meant for the future, which in her plans means home offices, mudrooms, and traditional kitchens. In a smaller ADU, each of those rooms competes for whatever floor space is left after the walls and hallways come out.

How Do I Budget Usable Square Footage Before Picking Rooms?

Do the math before you design, and think of the percentage as a placeholder until you have an actual:

• Start with the gross floor area the state and your town will, measured inside the exterior walls.
• Subtract 14 to 16 percent as a starting figure for interior walls and hallways.
• Make up the room list with what is left on the list and size each room for that number.
• Put the rooms in order of importance so the designer knows which rooms to protect first.
• Replace the allowance with the actual measurements once you have a plan for your layout.

Ranking is important because every time you have to redraw the plans it takes time. Gwendolen said it this way on the podcast: customization costs money and time. A ranked list gives your priorities to the designer before the first draft, so the plan starts from what you need most instead of from just the sketch.

Is a Custom ADU Plan Always the Right Answer?

We're a design-build company, so you'd think we would tell you to not bother drawing your own plan and let us do it. Keep the sketch. It shows how you want to live in the ADU, and it tells a designer more about your routine in a few minutes than any amount of questions will. The sketch only causes trouble when you think of it as a measured plan.

Also remember that the figures in this article come from a stock plan library rather than from us. A stock plan has already decided where the walls and hallways go, because that layout was drawn and measured before anyone bought it. If a stock layout works for your lot and your priorities, then it will work without paying for a redraw. And if your list still needs more floor than the cap will let you have after the subtraction, that's worth finding out before design starts rather than after.

Ready to See What Your Square Footage Will Really Hold?

Do the math first and then draw the design. A room list that's based on the floor area that's left after interior walls and hallways will hold up when the plan for your lot is drawn.