Portsmouth, NH · Progress Portsmouth

Thinking about an
accessory dwelling unit?

A step-by-step guide to understanding, planning, and building an ADU on your property.

✓ Updated · February 2026 · New state law in effect

What is an accessory dwelling unit?

An ADU is a small, self-contained apartment on the same lot as your home — with its own kitchen, bathroom, and entrance. It cannot be sold separately from the main house.

🏠
Attached
Addition or converted space connected to main house
🪵
Detached
Separate structure in backyard or on lot
🪜
Interior
Converted basement, attic, or garage

The rules just got easier. As of February 17, 2026, Portsmouth aligned its zoning with new state law. ADUs are now allowed by right in all single-family zones — no public hearing, no special permit. You submit a building permit and go.

💡 Why homeowners build them
  • Rental income to offset mortgage or property taxes
  • Housing for a family member (aging parent, adult child)
  • Long-term housing stability as needs change
  • Increase property value
🔑 One requirement you can't waive

The owner must live on the property — either in the main house or in the ADU — as their primary residence. This is an ongoing requirement, not just at permit time.

Is an ADU feasible for your property?

Answer these questions to get a quick read on whether your lot and situation are a good fit.

1 · Do you have a single-family home on your lot?
An ADU must be accessory to a single-family dwelling. Condos and multi-family buildings are not eligible.
2 · Will you live on the property — in the main house or ADU?
3 · What type of ADU are you considering?
A non-conforming structure is one that doesn't meet current setback or size rules — a pre-existing shed or garage, for example.
4 · How large are you thinking?
5 · Do you have at least one available off-street parking space beyond your own?
Current rules require 1 off-street space for ADUs over 500 sq ft, 0.5 for smaller ones. The city is actively working to relax this further.
Your quick assessment

What might it cost?

Construction costs vary widely based on what you're building and how. Use this estimator as a ballpark — get real quotes from licensed contractors before committing.

Select your ADU type:

Max allowed: 750 sq ft · 2 bedrooms
Estimated build cost
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Cost per sq ft
—
Potential gross rent
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Rough payback (yrs)
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Estimates based on $200–$350/sq ft range for Portsmouth-area construction. Rental estimate reflects Portsmouth median for studios/1-BRs ($2,300–$2,700/mo). These are not guarantees.
🏦 Financing options
  • HELOC — most common; draws on home equity, variable rate. Compare lenders.
  • Cash-out refinance — larger lump sum but replaces your mortgage rate.
  • Construction loan — draws in stages as work is completed.
  • NH Housing / community programs — ask Planning staff about any current programs.
📋 Don't forget permit fees

Building permit fees are based on construction value. Verify current fee schedule at the Planning & Sustainability office before budgeting. OpenGov accounts are required for online permit applications.

Portsmouth ADU rules at a glance

These rules are current as of February 2026 following Portsmouth's zoning update to comply with NH state law. Always verify with Planning & Sustainability before filing.

✓ Reflects Feb. 17, 2026 City Council vote
Max size
750 sq ft
Max 2 bedrooms
Where allowed
All SF zones
By right — no hearing
Permit required
Building permit
CUP no longer required
Owner-occupancy
Required
Main house or ADU
Parking — ≤500 sq ft
0.5 space
Practically 0–1 space
Parking — >500 sq ft
1 space
Ongoing discussion to eliminate
Max height (detached)
22 ft
Visually subordinate
Egress
1 entrance
Indoor or outdoor

Non-conforming structures — under the new state law, Portsmouth must allow conversion of an existing non-conforming structure into an ADU without requiring a variance. If you have an old garage or shed that doesn't meet current setbacks, it may still be convertible.

⚠️ What changed on Feb. 17, 2026
  • Conditional Use Permit (CUP) — eliminated
  • ADU-specific setbacks, height & footprint rules — removed
  • Architectural guidelines specific to ADUs — removed
  • Dual egress requirement — reduced to one entrance
  • Parking: streamlined to 0.5 space (≤500 sq ft) or 1 space (>500 sq ft)
🔄 Annual compliance

Each spring the City emails you through OpenGov to confirm owner-occupancy. Respond to keep your Certificate of Use active.

How to get approval and build

The process is now simpler than it used to be. Here's the full path from first question to Certificate of Occupancy.

Step 1
Talk to Planning first — free, no obligation
~15 minutes
Call or visit Planning & Sustainability to confirm your lot qualifies and get answers to any site-specific questions before spending money on plans. 603-610-7216.
Step 2
Hire a designer and prepare plans
Most ADU permits require architectural drawings showing floor plan, elevations, and site plan. For interior conversions, a licensed contractor may be able to prepare these. Detached structures or additions typically need an architect or designer.
Step 3
Create an OpenGov account and submit
Portsmouth's building permit applications are submitted through OpenGov. Upload your plans, pay the permit fee (based on construction value), and submit. No public hearing is required — staff reviews the application.
Step 4
Building permit review
Typical: 30–45 days
City staff reviews for zoning compliance and building code. They may request revisions. Once approved, you'll receive a building permit. You do not need to post a public notice sign under the new process.
Step 5
Construction and inspections
Your contractor builds the ADU. Inspections are required at key stages (framing, electrical, plumbing, insulation, final). Do not cover work before inspections are signed off.
Step 6
Sign and record the Owner-Occupancy Affidavit
Before receiving your Certificate of Occupancy, you must sign an affidavit confirming owner-occupancy. This is recorded with the county registry of deeds and remains on your property record.
Step 7
Certificate of Occupancy — you're done
Final inspection passes → Certificate of Occupancy is issued → your ADU is legal to occupy. You'll receive an annual email via OpenGov to reconfirm owner-occupancy.

"When the people find that out, they're very happy that they don't have to go through any process."
— Peter Stith, Assistant Planning Director, Feb. 17, 2026

Contacts & resources

Everything you need to take the next step.

City of Portsmouth
📞
Planning & Sustainability — free pre-application consult
603-610-7216 · 30–45 min, no obligation, recommended first step
→ 📋
ADU Handbook (46 pages)
portsmouthnh.gov → Planning & Sustainability → Accessory Dwelling Units · Note: handbook update in progress as of Feb. 2026
→ 🖥️
OpenGov — building permit applications
Create an account to submit plans and pay permit fees online
→
Housing help
🧭
Housing Navigator — Position currently vacant
Progress Portsmouth is hiring a new navigator. Check progressportsmouth.com for updates.
→
About this guide
ℹ️ Sources & accuracy

This guide reflects rules as of February 17, 2026 following Portsmouth City Council's zoning ordinance update. Cost estimates are indicative only ($200–$350/sq ft, local market conditions). Rental estimates are based on Portsmouth median rents as of January 2026. Always verify current fees and requirements with Planning & Sustainability before filing.

Progress Portsmouth