What Is an Accessory Dwelling Unit (ADU) in New Hampshire?

🏡 What Is an Accessory Dwelling Unit (ADU) in New Hampshire?

With rising housing demand across Central NH — including Plymouth, Campton, Ashland, Thornton, Wentworth, and Rumney — many homeowners are looking for ways to create more space or generate rental income.

One of the most popular options is building an Accessory Dwelling Unit (ADU).

Here’s what NH homeowners need to know.

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What Is an ADU?

An Accessory Dwelling Unit is a smaller, secondary unit built on the same lot as a single-family home.

It can be:

✔ A basement or attic apartment

✔ A garage conversion

✔ A detached cottage

✔ An in-law apartment

✔ A connected addition with a separate entrance

NH law requires all towns to allow at least one ADU per single-family property (RSA 674:71).

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Why NH Homeowners Build ADUs

ADUs are popular because they offer:

✔ Rental income

✔ Space for parents or adult children

✔ Guest housing

✔ Caregiver accommodations

✔ Increased property value

✔ Affordable housing options

In college towns like Plymouth, ADUs are especially valuable for rental opportunities.

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ADU Rules in New Hampshire

Local zoning still controls:

✔ Maximum size (often 750–1,000 sq. ft.)

✔ Parking requirements

✔ Owner-occupancy rules

✔ Attached vs. detached options

✔ Septic capacity requirements

✔ Lot size minimums

Most NH towns require a building permit and may require septic evaluation or upgrades.

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Septic Requirements for ADUs

In rural towns like Campton, Rumney, and Thornton:

✔ Your existing septic system must support the added bedroom(s)

✔ You may need a septic upgrade if capacity is exceeded

✔ The state must approve the septic design for any increase in flow

This is often the biggest limiting factor for ADUs.

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Q&A

Q: Can I Airbnb an ADU in NH?

A: Depends on your town. Some allow short-term rentals; others require long-term only.

Q: Does an ADU increase taxes?

A: Yes — because it adds livable square footage and value.

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Keywords

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Jim Johnson Real Estate

Phone: 857-249-7392

Serving Plymouth, Campton & Central New Hampshire

Brokered by Three Hills Real Estate Service