Londonderry NH ADU Guide: 2026 Rules & Costs

ADU law update (September 2026): Two more state changes just landed. As of August 31, 2026, HB 1079 lets you convert an existing structure into an ADU even if it sits in a spot that violates today's setbacks, without a variance and without losing its grandfathered status. And as of September 13, 2026, New Hampshire now defines tiny homes and yurts of 600 square feet or less as permanent dwellings that can serve as a single-family home or an ADU, so long as they meet the state building code. Stack those on top of HB 577, which has required every town to allow one ADU by right since July 1, 2025, and Londonderry homeowners have more room to work with than the town's 2015 ordinance suggests.

The short version for Londonderry

Londonderry has been friendlier to ADUs than most Southern NH towns for a while. Section 5.18 of the zoning ordinance, adopted back in 2015, allows one accessory dwelling on any lot with an owner-occupied single-family home, and it only takes a building permit, not a Zoning Board hearing. That's the good news. The catch is that the ordinance hasn't been rewritten since HB 577 passed, and the town's own consultants told the Planning Board in March 2026 that several of its ADU standards are now "more restrictive than RSA 674:72-73." If you own a home in Londonderry NH real estate and you've been told "no" on a detached unit or a size, it's worth reading on.

What Londonderry's ordinance says (Section 5.18)

• Owner-occupancy: You have to live in either the main house or the ADU when the permit is approved. HB 577 still allows towns to require this, so it stands.
• One ADU per lot, with full kitchen, bath, and sleeping quarters.
• Size: no more than 40% of the principal home's living area and no more than two bedrooms.
• Where it can go: inside the house, as an addition, attached to a detached garage, or as a free-standing structure. Detached units must be in the side or rear yard, and an interior ADU can't have a second front door facing the street.
• Parking: one space in the side or rear yard, with no new curb cut.
• Design: consistent with the main house in appearance, colors, and materials, and all normal setbacks apply.
• Utilities: if you're on a well and septic, NHDES rules govern capacity.
• No condo-izing: the ADU can't be split off as separate ownership.

That side-or-rear-yard rule is real and enforced. In August 2026 a Wilson Road homeowner had to go to the Zoning Board for a variance just to put an ADU in a front yard. Plan your siting around it.

Where state law overrides the town

The 40% size cap is the big one. HB 577 says a town can't limit an ADU to less than 750 square feet, and the default ceiling is 950 unless the town allows more. On an 1,800-square-foot ranch, 40% is 720 square feet, which is below the state floor. On a 2,400-square-foot colonial it's 960. So for most modest Londonderry homes, the state's 750-square-foot floor is your real minimum, not the town's percentage. HB 577 also blocks towns from piling ADU-only setbacks or extra design hurdles beyond what a single-family home already faces, and HB 1079 now protects conversions of existing garages, barns, and workshops that don't meet current dimensional rules. Londonderry's audit flagged exactly that gap: "missing exemption from dimensional standards for ADUs in existing non-compliant structures." Draft amendments are in the works, but until the ordinance is rewritten, bring the statute (RSA 674:71-73) to the Building Division and get the answer in writing before you spend a dollar.

Do the numbers work?

Londonderry rents are firm. As of September 2026 the median rent is about $2,150, one-bedrooms run roughly $1,750, two-bedrooms about $2,300, and single-family rentals average $2,700. A finished ADU in a good Londonderry neighborhood realistically brings $1,600 to $2,200 a month depending on size, finish, and whether it's a true detached unit.

On the cost side, converting existing space (a garage, walk-out basement, or barn) generally runs $80K to $150K because you reuse the foundation, walls, and roof. A new detached cottage lands in the $150K to $300K+ range. A code-compliant tiny home, now legal statewide as an ADU, is being quoted by NH builders at $100K to $200K. At $2,000 a month, a $150K conversion covers a typical 20-year home-equity payment with cushion to spare, and it adds appraisable square footage whether you rent it out or house a parent. Just note that Londonderry's ordinance is built around long-term tenants, so don't underwrite it as a short-term rental.

Not sure Londonderry is the right town for this? Our guide to living in Londonderry covers the neighborhoods where larger lots and detached garages are common, and if you're comparing across the border, the Manchester NH ADU guide shows how a city with a brand-new ordinance handles the same rules.

Quick answers: Londonderry NH ADUs

Can I build a detached ADU in Londonderry? Yes. The town has allowed free-standing accessory dwellings since 2015, and HB 577 guarantees it by right on any lot where a single-family home is allowed. It must sit in the side or rear yard.

Do I need a Zoning Board hearing? Not for a conforming ADU. Londonderry handles it through a residential accessory dwelling permit at the Building Division.

How big can my Londonderry ADU be? The town says 40% of the main home's living area, but state law sets a floor of 750 square feet and a default ceiling of 950. Where the town's percentage falls below 750, the state number controls.

Can I rent it to a non-family member? Yes. Londonderry has no family-only restriction, and the town counts every ADU as workforce housing. Owner-occupancy of one of the two units is still required.

Can I convert my old garage even though it's inside the setback? As of August 31, 2026, HB 1079 says yes for existing structures, without a variance. Confirm the details with the Building Division since the town's text hasn't caught up yet.

Want to know what your lot can handle?

Every Londonderry lot is different: septic capacity, side-yard width, whether the garage is already attached, and whether that 40% number or the state floor applies to your house. The Phinney Team will give you a free property feasibility read. Send us your address and what you're picturing and we'll tell you what's realistic and what it's likely worth when it's done. Want to scope it yourself first? Grab our ADU Feasibility Checklist and walk through it in ten minutes. Reach out anytime at teamphinney.com. Aaron Phinney, The Phinney Team at Keller Williams Realty Metropolitan, Bedford, NH.

This guide is general information, not legal advice. Londonderry is actively drafting amendments to its ADU ordinance, and HB 577, HB 1079, and the new tiny-home law are still being interpreted town by town. Always confirm current requirements with the Town of Londonderry Planning Division and Building Division before you design or build.

Sources: Town of Londonderry Residential Accessory Dwelling Permit Application and Section 5.18 · Londonderry Planning Board, 2025 NH RSA Audit and Housing Strategy Direction (March 2026) · Londonderry ZBA public hearing notice, August 19, 2026 · NH Municipal Association, The Revised Law on Accessory Dwelling Units 2025 · NHPR on HB 1079 (July 2026) · NHPR on the NH tiny-home law (August 2026) · Zumper Londonderry rent report (September 2026)