The Apartment That Is Not Legal: Buying a House With an Unpermitted Unit

Greater Boston is full of houses with more homes in them than their paperwork admits: a "single-family with in-law" that has its own kitchen and its own tenant, a basement apartment in a two-family, a third floor let out for years. The listing usually says something careful — "in-law potential", "bonus space", "income opportunity" — and the price usually assumes the rent.

If the town's zoning and building records do not recognise the extra unit, the house is legally what the records say it is. The lender will generally underwrite it that way. The appraiser may give the unit little or no value as a separate home. The insurer wrote a policy on something different. And the person living in it is still a tenant with every protection Massachusetts gives tenants. None of that makes the house a bad purchase. It does mean you should pay for what is legal, plus a realistic view of what it would cost to make the rest legal.

How to tell whether the extra unit is legal

Separate entrances, separate meters and a separate mailbox prove nothing; they are what every illegal unit has. The evidence is in the town's records, and researching a house before you offer shows where each one lives.

The assessor's use code. Massachusetts assessors classify residential property by use: a single-family, a two-family and a three-family each have their own code. A "two-family" carried as a single-family on the card is the first sign.
Building permits for the conversion. A legal second unit was created under a building permit, with electrical and plumbing permits, and closed out with a final inspection.
A certificate of occupancy, or its equivalent in the town's records, for the number of units.
Zoning relief. A unit allowed by special permit or variance will have a recorded decision, often with conditions — owner occupancy, for example — that bind future owners.

If the records show one unit and the house has two, ask the seller directly, in writing, how the second came about. Then ask the town's building commissioner what they would require.

What the lender and the appraiser will do with it

A lender lends against the property as it legally exists. In practice that usually means:

The rent from the unit generally will not count as income for qualifying. If your budget depends on it, it may not be there on paper.
The appraiser values the house as its legal use, and may give the extra kitchen and bedroom some value as space while giving little or none as a rentable unit. When the price assumes the rent, the appraisal can come in short. What happens if the appraisal comes in low covers the options.
Some loans will not proceed at all where the property's use does not conform to zoning and the appraiser says so. Ask your lender at pre-approval, not after the appraisal.

On a property marketed as a two-family that is legally a single-family, the type of loan itself changes. Owner-occupied two- to four-family lending has its own rules, which buying a two- or three-family covers.

The fire-safety question is the real one

Paperwork aside, the reason units need permits is that people sleep in them. Unpermitted apartments in basements and attics commonly lack the things that keep their occupants alive in a fire:

A second means of escape, or an egress window large enough to climb out of.
Fire separation between the units: rated walls and ceilings, self-closing doors.
Interconnected smoke and carbon monoxide alarms in each unit.
Adequate ceiling height, which many basements cannot meet.

Your inspector will flag what they can see, but they cannot certify compliance. The insurance consequence follows from the same facts: a policy written for a single-family house with an undisclosed rental unit can make a claim harder. Tell the insurer what the property actually is when you buy it. Home insurance in Massachusetts covers what to ask.

A tenant in an unpermitted unit is still a tenant

This surprises buyers more than anything else. The fact that the unit should not exist does not remove its occupant's rights.

The security deposit statute applies, with its requirements for holding and returning the money.
The lead law applies if a child under six lives there in a home built before 1978. The lead law explains the owner's obligation.
Removing a tenant requires the court process, whatever the unit's legal status. A new owner who intends to stop renting the unit inherits the tenancy and the process.

If the unit is occupied, ask for the lease, the deposit records, and the tenant's own written confirmation of the terms, and read selling a house with tenants, which covers the same facts from the other side.

Can it be made legal? The 2024 ADU law changed the answer for some houses

For a second unit in a single-family house, often yes, and more easily than before.

Since February 2025, the Affordable Homes Act has allowed one accessory dwelling unit by right in single-family zoning districts across Massachusetts, up to 900 square feet or half the floor area of the main house, whichever is smaller. Towns may apply reasonable regulations, such as site plan review, but may not require a special permit for that first unit. In-law apartments and ADUs covers what towns may and may not demand.

Zoning is only half of it. The unit must still meet the building code, and that is where a basement apartment with a six-foot ceiling and no egress window becomes expensive or impossible.

A third unit in a two-family is harder. It is a change in the number of dwellings, not an accessory unit, and usually needs zoning relief that is genuinely discretionary. In some towns the new multifamily districts created under the MBTA Communities Act may help a property inside them; most are not.

How to price a house whose extra unit may not survive

Start from the legal use. What do comparable legal single-families, or legal two-families, sell for? How to read a comp applies here.
Add the realistic value of legalising, minus its cost and the chance it fails.
Do not pay for rent you cannot legally collect, and do not count on it to qualify for the loan.
Give yourself time. Ask for long enough before the purchase and sale agreement to talk to the building commissioner and, if it matters, a zoning attorney. Ask for written seller representations about the unit's history.

The buyer's guide covers the wider sequence, and a written valuation is how a seller gets the same question answered before listing.

General information as of 2026, not legal advice. Zoning, building code and ADU rules are applied town by town; the building commissioner and a Massachusetts real estate attorney are the authorities on a specific property.