Looking for a basement contractor in Portsmouth, NH? Describe your project once and up to three basement contractors who work in Portsmouth and Rockingham County get back to you with a quote. It's free and there is no obligation.
Portsmouth is home to about 22,938 people (2024 Census Bureau estimate), a growing community (+4.5% since 2020). With 1,465 people per sq mi, it is a suburban area. Full basements are common here and often the cheapest way to add living space.
Portsmouth at a glance
- Population (Census Bureau estimate, 2024)
- 22,938 (+4.5% since 2020)
- Rank in New Hampshire
- 7th of 13 cities and towns
- Land area
- 16 sq mi
- Population density
- 1,465 people per sq mi (suburban)
- Type
- City (Portsmouth city)
- Main county
- Rockingham County
What to check before hiring in Portsmouth
- Moisture first: waterproof before finishing.
- Egress: in New Hampshire, a basement bedroom needs an escape opening.
- Radon: test before you finish.
- Price drivers: size, bathroom, ceiling height, finishes and egress.
Useful official resources
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ICC: International Code Council
Official information and guidance.
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EPA: radon
Official information and guidance.
Services available in Portsmouth
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Basement finishing
Framing, insulation, electrical, drywall and flooring of a basement, with permit.
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Basement bathroom or rental unit
Bathroom, kitchenette or legal basement unit with egress and plumbing.
Frequently asked questions in Portsmouth
How much does basement finishing cost in Portsmouth, NH?
It depends on size, bathroom and finishes. Describe your basement: contractors who serve Portsmouth reply with a quote.
How long does it take?
Often several weeks.
Can I rent it out?
Only if local rules allow an accessory unit; check zoning.
Which basement contractors will contact me?
Up to three independent basement contractors who serve Portsmouth and the surrounding area, chosen for the service you need. Your details are shared only with your consent.
Source: U.S. Census Bureau, Population Estimates Program (Vintage 2024) and Gazetteer Files (public domain), imported on 10/31/2026. Figures describe the incorporated place or county as published; they are shown for information only.