Designing an ADU for Aging in Place: Single-Level Living, Wide Doors, Zero Steps

Designing an ADU for aging in place comes down to three commitments: put everything on one level, eliminate every step, and make every doorway wide enough for whatever mobility looks like ten years from now. Get those three right — and add a curbless shower, lever handles, and some hidden future-proofing behind the walls — and you have a home a parent can live in comfortably at 68 and safely at 88, without a renovation in between.

New Hampshire makes this easier than it used to be. Since July 1, 2025, state law (RSA 674:72, amended by House Bill 577) allows one ADU — attached or detached — by right on any lot where a single-family home is permitted, with no special exception or variance for the ADU itself. You still need a normal building permit and still meet local setbacks, septic, and building code. But the design is yours to shape, and shaping it for aging in place is the smartest version of the project. Here's how.

Why is single-level living the foundation of the design?

Stairs are the single biggest reason older adults leave homes they love. So the aging-in-place ADU starts with a simple rule: bedroom, bathroom, kitchen, laundry, and living space all on one floor, with no sunken rooms, no split levels, and no "just two steps down to the den."

A few layout principles do most of the work:

• Compress the circulation. Long hallways waste square footage and create distance a walker or wheelchair has to cover. An open plan with the bedroom and bathroom directly off the main living space keeps every daily destination a few steps apart.
• Put laundry inside the unit. A stackable washer and dryer near the bedroom means no trips to the main house and no baskets carried across a yard in a Nashua January.
• Keep the bedroom and bathroom adjacent. The most common nighttime trip in the house should be the shortest one, and it should be well lit the whole way.

A one-story detached cottage does this naturally. If you're converting space instead — a garage bay, a wing of the house — the same rule applies: the finished unit itself should live entirely on one level. And state law protects the garage conversion even if the existing garage doesn't meet today's setbacks. Our guides walk through the attached-versus-detached decision in more depth.

What makes an entry truly zero-step?

"Zero-step" means a person can go from the driveway to the living room without lifting a foot over anything — no porch step, no raised threshold, no lip at the door.

• Grade the path, don't ramp it. When the site is planned early, the walkway can rise gently with the yard so the entry lands flush. A gradual graded path looks like landscaping; a retrofit ramp looks like medical equipment. Both work, but one preserves dignity and resale appeal.
• Cover the entry. A roof over the door means no fumbling for keys in the rain and less ice at the exact spot where footing matters most — a real consideration in a New Hampshire winter.
• Flush thresholds everywhere. The front door, the patio slider, the shower: every transition in the unit should be rollable. Threshold height is a detail to put in writing with your builder, because standard doors ship with a lip.
• Light the approach. Motion-activated lighting along the path and at the door is cheap insurance against the falls that happen ten feet before the house.

Why 36-inch doors and lever handles?

Standard interior doors in many builds are too narrow for a wheelchair, and tight for a walker with someone assisting. Specifying 36-inch doors throughout — every door, not just the front one — costs little at framing time and removes the single most common barrier retrofits have to fix later.

Pair the wide doors with:

• Lever handles instead of knobs. Arthritis makes a round knob a daily fight. A lever opens with a wrist, an elbow, or a full grocery bag.
• Pocket or barn doors where swing space is tight. A door that slides never blocks a walker mid-turn in a small bathroom.
• Generous clear space at turns. Wherever a hallway meets a doorway, give the turn room to happen. Your designer should plan actual turning clearances for a wheelchair at the bathroom and bedroom entries, not just straight-line widths.

What does an aging-in-place bathroom look like?

The bathroom is where most in-home injuries happen, so it gets the most design attention:

• Curbless (roll-in) shower with a gentle slope to the drain, a fold-down bench, and a handheld sprayer on an adjustable bar.
• Blocking in the walls — solid lumber behind the tile around the shower and toilet, so grab bars can be installed anytime, anywhere, and hold real weight. This is the cheapest future-proofing in the whole build and it's invisible until needed.
• Comfort-height toilet and a vanity with knee space or the plumbing set so one can be swapped in later.
• Non-slip flooring and bright, even lighting, including a lit path from bed to bathroom for 2 a.m.

None of this reads as "accessible" to a visitor. Done well, it reads as a nice modern bathroom — which is exactly the point.

Can you fit all this within New Hampshire's size caps?

Yes, comfortably. Under RSA 674:72, a town may cap ADU size, but never below 750 square feet — and a thoughtfully planned single-level, one-bedroom layout with a full kitchen, laundry, and an accessible bathroom fits within that floor. Universal design is mostly about where walls and doors go, not about needing more of them.

Many towns allow more room to work with. Amherst caps accessory apartments at 1,100 square feet (confirmed with the town), and Meredith allows up to 1,200 square feet under a formula tied to the main home's size (also confirmed with the town). Since a single-level unit puts its whole footprint on the ground, your buildable area — setbacks, lot coverage, septic capacity — matters more than in a two-story design, which is exactly the kind of detail our town pages in Hillsborough, Belknap, and every other covered county break down. The full statewide picture is on our NH ADU law page.

Two more state-law points work in your favor. A town can require at most one additional parking space, and the owner chooses whether it sits on-site or off-site — and a zero-step path from that space to the door should be part of the site plan. And if the town requires owner-occupancy of the property, the owner chooses which unit to live in. That flexibility is its own future-proofing: plenty of homeowners plan to move into the ADU themselves someday and pass the main house to the next generation.

Is an aging-in-place ADU worth the money?

A typical New Hampshire ADU build runs $275,000 to $400,000+ depending on design, and the aging-in-place features above are largely a matter of planning rather than premium materials when they're in the drawings from day one — wide doors, blocking, and a graded entry cost far less specified up front than retrofitted later. Set against assisted living, which commonly cited industry surveys put at $70,000 to $100,000+ per year in this region, a $300k build that keeps a parent independent, private, and thirty feet away has a payback story few other projects can match. And when its aging-in-place chapter ends, a step-free, single-level cottage is the most rentable, most sellable unit on the market — every future tenant ages too.

Design it once, for the whole arc of a life, and the backyard cottage becomes the last move a parent ever has to make.