ADU Builders in New Hampshire
Ryan & Greene Construction designs and builds accessory dwelling units and in-law suites across Rockingham and Hillsborough counties. Veteran-owned, licensed, and insured, with 30+ years of additions, conversions, and multi-generational living projects in New Hampshire.
Four ways to add an ADU to a New Hampshire home
An accessory dwelling unit is a self-contained living space with its own kitchen, bathroom, and entrance. Which approach fits depends on your lot, your existing structure, and your town's zoning.
Attached ADU addition
A new wing built off the existing house with its own entrance, kitchen, and bath. Shares the main structure and utilities, which keeps costs down.
Typically $150,000 - $320,000
Interior conversion (basement or upper floor)
Converting existing conditioned space into a legal dwelling unit. Ceiling height, egress windows, and fire separation drive feasibility.
Typically $85,000 - $180,000
Garage conversion or above-garage suite
Reuses an existing footprint. Insulation, floor build-up, heating, and a code-compliant stair are the main scope items.
Typically $95,000 - $220,000
Detached ADU (cottage)
A standalone small home on the same lot. Requires its own foundation and utility runs, and often septic review on unsewered lots.
Typically $200,000 - $400,000
2026 ADU and in-law suite costs in Southern NH
Ranges below are all-in, including design, permits, structure, mechanicals, and finishes. Site conditions, septic capacity, and utility runs move the number more than anything else.
| Project | Typical cost | Notes |
|---|---|---|
| Basement in-law conversion (500-700 sq ft) | $85,000 - $180,000 | Egress windows, ceiling height, and moisture control decide feasibility |
| Garage conversion (400-600 sq ft) | $95,000 - $220,000 | Floor build-up, insulation, heating, and a separate entry |
| Attached ADU addition (500-800 sq ft) | $150,000 - $320,000 | New foundation and roof tie-in, shares main utilities |
| Detached ADU cottage (600-900 sq ft) | $200,000 - $400,000 | Standalone structure, separate utility runs, possible septic upgrade |
| Septic expansion (when required) | $18,000 - $45,000 | NHDES approval for added bedrooms on unsewered lots |
| Design, engineering, and permits | $6,000 - $20,000 | Plans, structural review, town fees, and any zoning hearing |
Want to model your own numbers? Use the home addition cost calculator and set the square footage and finish level to match your plan.
New Hampshire ADU rules, in plain language
Attached ADUs are allowed statewide. RSA 674:71-73 requires New Hampshire municipalities to permit one attached accessory dwelling unit in any zoning district that allows single-family homes.
2025 reform (HB 577) went further by expanding what towns must allow and limiting the extra conditions they can impose, which has made detached and larger units viable in more communities.
Owner occupancy and family-only limits have been narrowed, but individual towns still vary. We confirm your specific ordinance before design starts.
Septic capacity is the common blocker. On unsewered lots, NHDES approval for added bedrooms often drives both timeline and budget. We check this first.
Egress, fire separation, and parking are the details that fail inspections. Attached units generally need rated separation between the dwelling units and an independent exterior entrance.
Permits and review typically take 6 to 14 weeks depending on whether a zoning board hearing is required.
For a deeper walkthrough, read our in-law suite and ADU guide for Southern New Hampshire.
Our design-build process
- Free on-site consultation: we review the lot, structure, septic, and your goals.
- Feasibility and zoning check with your town before you spend money on design.
- Design and fixed-scope proposal with a line-item budget, not a vague allowance sheet.
- Permitting, engineering, and any zoning board representation needed.
- Construction with a single point of contact and a published schedule.
- Final inspections, certificate of occupancy, and written warranty.
ADU builders in NH, frequently asked questions
Planning an in-law suite or ADU in New Hampshire?
We serve Plaistow, Atkinson, Hampstead, Salem, Windham, Derry, Londonderry, Exeter, Manchester, Bedford, Nashua, and the surrounding Merrimack Valley.