Can My Mom Live in an ADU on My Property in NH?

Yes. Your mom can live in an accessory dwelling unit on your property anywhere in New Hampshire, and since July 1, 2025, state law guarantees your right to build one. Under RSA 674:72 (amended by House Bill 577, 2025 Chapter 197, effective July 1, 2025), every town must allow one ADU, attached or detached, by right on any lot where a single-family home is permitted, with no special exception, variance, or conditional-use permit for the unit itself.

Here's the part most families miss: the same law also means the ADU doesn't have to be for your mom. Towns can no longer require the occupant to be a family member at all. So the backyard cottage you build for a parent today isn't legally locked to that arrangement forever, which matters more than it might seem right now.

Is it legal to build an ADU for an aging parent in New Hampshire?

It is, in every town. "By right" means the town cannot say no to the ADU itself: you apply for a normal building permit, and your project meets the town's reasonable standards (setbacks, lot coverage, septic capacity, building and fire codes). What no town can do anymore is send you to a zoning board to plead your case for permission to have the unit exist.

A few statewide protections are especially relevant when the plan is a parent moving in:

• Attached or detached, your choice. An in-law apartment over the garage, an addition with its own entrance, or a standalone backyard cottage all qualify.
• A town may cap ADU size, but never below 750 sq ft. That's enough for a comfortable one-bedroom, single-level home.
• Towns can't limit the ADU to one bedroom, so a second bedroom, den, or caregiver room stays on the table.
• At most one extra parking space can be required, and the owner chooses whether it goes on-site or at a legal off-site spot.
• Converting an existing garage is allowed even if the garage doesn't meet today's setbacks. For many families, that's the fastest path to a parent's unit.
• Separate electrical service or meter is your choice. The town can't force the ADU to share the main home's panel.

The full statewide picture is in our plain-English guide to the NH ADU law.

Does the ADU have to be occupied by a family member?

No, and this is the piece the 2025 law changed most decisively for families. Towns cannot require any familial relationship between the people in the main house and the people in the ADU. Family-only occupancy rules, which used to be common in New Hampshire "in-law apartment" ordinances, are gone as a legal requirement statewide.

Be aware that many towns' posted ordinances haven't caught up. Some still carry family-only language, deed-restriction requirements tied to relatives, or staff practices that treat a "family unit" differently from a "rental unit." Where that language conflicts with RSA 674:72, the state law controls. But a homeowner reading the town's PDF, or a counter clerk quoting it, may not know that. Our town guides read each ordinance side-by-side with the state law and mark exactly which clauses are superseded.

Why does this matter if your mom is the whole point? Because life changes. If she later needs a higher level of care, or the unit sits empty, you aren't stuck with a deed-restricted "relatives only" apartment. You can generally rent it long-term like any other dwelling, subject to your town's ordinary rules. Which brings us to the one requirement that does commonly survive.

What does "owner-occupancy" mean for our arrangement?

New Hampshire law allows a town to require that the owner live on the property, but the owner picks which unit. That flexibility is written for exactly this situation: nothing stops you from living in the ADU yourself someday and moving a family into the main house, or the reverse.

Real examples from towns we've verified:

• Hollis requires that either the principal dwelling or the ADU be owner-occupied (it doesn't dictate which), and even offers a hardship escape valve: an owner who temporarily can't comply, say a job relocation or a medical or family emergency, may seek time-limited relief from the Board of Adjustment.
• Atkinson requires owner-occupancy of the house or the ADU, with both units staying in common ownership.
• Manchester likewise requires that either unit be owner-occupied and that both remain under common ownership.
• Amherst is the cautionary tale: its posted accessory-apartment section contains no owner-occupancy clause, yet when we confirmed with the town, staff described a residency requirement in practice. Ask before you assume.

The pattern: owner-occupancy of the property is common and legal; owner-occupancy of a specific unit is not something a town can dictate. For a mom-in-the-ADU plan, you'll almost always satisfy this automatically. You live in the main house, she lives in the ADU, done.

Is an ADU a good alternative to assisted living?

Financially, it's a serious conversation. A typical New Hampshire ADU build runs $275,000–$400,000+ depending on design, while assisted living in NH commonly runs $70,000–$100,000+ per year according to commonly cited industry surveys. A $300k build is real money, but it's an asset on your property, not a recurring bill, and within a relatively short stay the comparison starts tilting hard toward the ADU. It also keeps your parent close, private, and independent, on one level, steps from family.

On paying for it: a HELOC, home-equity loan, cash-out refinance, or construction loan are the standard tools, and some lenders will count projected ADU rent toward qualifying. That's worth asking a local lender who knows ADUs. Some families also share costs with the parent, since the alternative is often selling her home to fund care anyway. Design-wise, the statewide 750 sq ft protected size supports a genuine single-level, zero-step, one-bedroom-plus home: aging-in-place from day one.

What happens if Mom moves out later?

The arrangement stays flexible, because the law never tied the unit to her:

• Long-term rental is generally available like any dwelling, subject to your town's normal rules. If your town requires owner-occupancy, you still just need to live in one of the two units.
• Family rotation works too: adult kids, a caregiver, you yourself downsizing into the ADU while renting the main house.
• Short-term rental is the exception to check. Some towns regulate short-term rentals as a separate use with their own permits and limits, so don't assume the ADU rules cover it.

Check your town's page for the specifics (size caps, setbacks, septic requirements, and every place the posted ordinance is superseded by the 2025 law), starting from our town guides or your county page, like Hillsborough County or Rockingham County.

The law already said yes to your mom. It quietly said yes to whatever comes after, and that's what makes this the rare family decision that's also a sound one.