How Much Value Does an ADU Add to a New Hampshire Home?

A well-built, permitted ADU adds real value to a New Hampshire home, because it adds the two things appraisers and buyers consistently pay for: finished living space and the potential for rental income. How much value depends on your town, your local sales comps, and how well the unit is documented — and anyone quoting you an exact percentage for New Hampshire is guessing, because the data here is still young.

That last part matters. Detached ADUs have only been legal by right statewide since July 1, 2025, under RSA 674:72 as amended by House Bill 577. Most NH neighborhoods simply don't have a long track record of ADU sales yet. So instead of a made-up number, here's how the value actually gets created, how an appraiser will look at it, and what you can do to make sure the value shows up when it counts.

Where does the value actually come from?

Three places, in roughly this order:

• Finished square footage. An ADU is permitted, heated, code-compliant living area — a full extra dwelling with a kitchen and bath. That is a different animal from a finished basement or a bonus room, and it's the most durable driver of appraised value.
• Income potential. A legal second unit can be rented long-term. Even if you never rent it, the option is worth something to the next buyer, and it widens your buyer pool to include house-hackers and multigenerational families.
• Flexibility. Housing an aging parent at home instead of in assisted living — which commonly cited industry surveys put at $70,000–$100,000+ per year in this region — is a financial benefit that never shows up on an appraisal but absolutely shows up in a family's budget.

The first two are what an appraiser can measure. The third is why many owners build even when the appraisal math alone wouldn't justify it.

How do appraisers put a number on an ADU?

Residential appraisers generally lean on three tools:

1. Sales comparison. Find recently sold homes nearby that also have an ADU and see what the market paid. This is the preferred method — and the hardest one in New Hampshire right now, because ADU comps are scarce in most towns.
2. Income approach. Estimate what the unit would rent for and capitalize that into value. More common for investment property, but a legal, rentable second unit gives the appraiser something concrete to work with.
3. Cost approach. What it would cost to build the improvement, minus depreciation. Useful as a sanity check, but appraisers know cost and market value are not the same thing.

When comps are thin, appraisers get conservative. That's not hostility toward ADUs — it's how the profession handles uncertainty. Expect the appraised contribution of a brand-new ADU to be a judgment call, and expect it to improve as more ADU-equipped homes sell in your area and the comp pool deepens.

Will the appraisal capture the full cost of the build?

Not necessarily — and it's better to know that going in. A typical New Hampshire ADU build runs $275,000–$400,000+ depending on design, site work, and utility connections. If you spend $300,000 on a detached unit, the appraisal the day after completion may or may not move by $300,000. Sometimes it will. Sometimes the market hasn't caught up yet, especially in towns where yours is the first ADU an appraiser has seen.

Two things soften that:

• Cost and value converge over time. As ADU sales accumulate post-2025, comps get easier and appraisals get more confident.
• The income doesn't wait for the appraisal. Rent starts offsetting the build cost from the first lease, whatever the appraised number says. If you're weighing the spend itself, our guides pair this question with a full breakdown of what an ADU costs to build in New Hampshire.

Reframed properly, an ADU is less like a kitchen remodel and more like buying a small rental property that happens to sit in your backyard — you evaluate it on income and resale, not resale alone.

Does rental income count toward value and financing?

Often, yes — in two different ways. At appraisal, a documented market rent gives the income approach something to anchor on. At financing, some lenders will count projected ADU rent when qualifying you for a construction loan or refinance, which can meaningfully change what you can borrow. Policies vary widely, so ask your lender — ideally a local lender who knows ADUs — how they treat projected rent before you design around it.

One New Hampshire-specific note: a town may require that the owner live on the property, but under state law you choose which unit you occupy. You can live in the ADU and rent the main house — a combination that sometimes rents better and is worth modeling both ways.

How do town rules change the value math?

State law sets the floor: every town must allow one ADU by right where single-family homes are permitted, and a town may cap ADU size but never below 750 sq ft. Above that floor, town choices directly shape how much value you can build:

• Amherst allows an accessory apartment up to 1,100 sq ft (within 70% of the main home's heated floor area), confirmed with the town.
• Hollis, by contrast, caps ADUs at 800 sq ft, also confirmed with the town.

Same county — see our Hillsborough County guide — but a 300 sq ft difference in maximum finished space, which flows straight into what an appraiser can credit. Bedroom counts, septic capacity, and setbacks vary town to town the same way. Before you assume a size, check your town's actual cap in our town guides.

How do you protect your appraised value?

Value that isn't documented is value the appraiser can't give you. The checklist:

• Permit everything. An unpermitted unit can subtract value and kill financing. Keep the building permit, inspections, and certificate of occupancy in one folder.
• Make it a legal ADU, not a "bonus room with a sink." A full kitchen, proper egress, and its own address or unit designation are what separate countable living area from ambiguity.
• Document the rent. A signed lease or a written market-rent analysis gives the income approach real inputs.
• Feed the appraiser comps. If ADU homes have sold anywhere nearby, your agent or builder can supply them — appraisers can consider comps they're handed.
• Build to the neighborhood. Quality consistent with the main house appraises better than either a bare-bones box or an over-improved showpiece.

The honest answer to the title's question is that nobody can hand you a reliable New Hampshire percentage yet — but the direction is not in doubt. You're adding a second legal dwelling to a state that just made them a right in every town, in a housing market that is chronically short on exactly this kind of unit.

Build it permitted, document it well, and let the square footage and the rent checks make the argument for you.