OWNERSHIP AND LIFE EVENTS

Does homeowners insurance cover an ADU? Attached or detached, family member or tenant

Coverage A of the ISO HO-3 sample policy treats an accessory dwelling unit built into or onto your house as part of the dwelling, even when a tenant lives in it. A detached unit is insured only under Coverage B, other structures, capped at 10 percent of the dwelling limit, and the sample excludes it entirely once it is rented to someone who is not a tenant of the main house.

Disclaimer: Published averages and benchmark ranges, not a quote. This page is not insurance advice. Your premium will differ based on your home, location, claims history, and insurer. Flood insurance is separate from a standard homeowners policy.

Massachusetts shows how fast the question is arriving at insurers. Its housing agency counted 1,639 ADU applications and 1,224 approved units in its survey of cities and towns for 2025, the first year a state law let owners build a unit of up to 900 square feet by right in single-family zoning districts. Each of those owners holds a homeowners policy written for one family in one house, and the policy's words, not the zoning permit, decide what a fire in the new unit pays.

Two words in the policy sort every ADU: attached and rented

The HO-3 sample is an Insurance Services Office form copyrighted in 1999, and it splits buildings into two coverages. Coverage A covers "the dwelling on the 'residence premises' shown in the Declarations, including structures attached to the dwelling." Coverage B covers "other structures on the 'residence premises' set apart from the dwelling by clear space," and it adds that structures "connected to the dwelling by only a fence, utility line, or similar connection" still count as other structures. A covered breezeway or a shared wall puts a unit on the Coverage A side. A sewer line and a walkway do not.

The second sorting word is in Coverage B's own exclusion list. Coverage B does not cover "Other structures rented or held for rental to any person not a tenant of the dwelling, unless used solely as a private garage." There is no matching rental exclusion in Coverage A. So the same tenant, paying the same rent, is inside the building coverage when the unit shares a wall with the house and outside it when the unit sits twenty feet away in the yard.

Two other definitions keep an attached rental inside the form. The sample's "residence premises" means "the one family dwelling where you reside" or "the two, three or four family dwelling where you reside in at least one of the family units," and it "also includes other structures and grounds at that location." Turning a single-family house into a two-unit house, with you living in one unit, keeps it within that definition. Moving out of the main house and renting both units does not, because the policy is written around the place where you reside. That is the point where the question becomes landlord insurance rather than an ADU question, and our guide to landlord insurance versus homeowners coverage walks through the switch.

A third clause sits behind the rental one. Coverage B also excludes "Other structures from which any 'business' is conducted," and the sample defines business broadly, as a trade or "any other activity engaged in for money or other compensation," with an exception for small activities that pay an insured no more than $2,000 in the 12 months before the policy period. An ADU used as a paid studio, a therapy office or a workshop hits that exclusion even with no tenant at all; the home business coverage guide covers that case.

The ADU decision table under the HO-3 sample

The table maps each common arrangement to the clause that decides it. Where the policy's words leave room for two readings, the result column says so and names the words that create the doubt, because those are the cells to put to the insurer in writing.

The table shows one sharp line and two gray zones. The sharp line is the detached rental: once rent is paid by someone who does not rent the main house, the sample's Coverage B simply does not reach the building. The gray zones are family arrangements. A parent living in a backyard cottage may or may not be a resident of your household in the policy's sense, and the answer changes whether their belongings and their liability fall under your policy. A relative who pays rent sits inside the plain words of the rental exclusion, because nothing in B.2.b turns on who the renter is. Insurers handle both differently, and a written answer from yours is worth more than any general rule.

The phrase "a tenant of the dwelling" has one narrow use. If the person renting the cottage also rents part of the main house, for example a caretaker who rents a room and uses the cottage as a workshop, the exclusion's words do not apply to them. That arrangement is rare for ADUs, and it should not be stretched to cover a tenant who rents only the cottage.

A tenant in the attached unit: what changes even though the building stays covered

Coverage A keeps an attached rental's walls, roof and systems inside the policy. Several smaller clauses then start to bite, and together they explain why an insurer wants to hear about the lease.

The tenant's belongings are not yours to insure. Coverage C, personal property, does not cover "property of roomers, boarders and other tenants, except property of roomers and boarders related to an 'insured'." A tenant's sofa destroyed in a kitchen fire is the tenant's loss unless the tenant has a renters policy. The HO-4 renters policy guide is the one to send a new tenant.

Your own furnishings in the unit have a small limit. Additional Coverage E.10, Landlord's Furnishings, pays "up to $2,500 for your appliances, carpeting and other household furnishings, in each apartment on the 'residence premises' regularly rented or held for rental to others," for Coverage C perils other than theft. The same Coverage C exclusion list removes "property in an apartment regularly rented or held for rental to others by an 'insured'" except as that $2,500 provides. A furnished unit whose appliances would cost, say, $6,000 to replace is underinsured on day one.

Theft from the rented part is excluded. The theft peril does not include theft "from that part of a 'residence premises' rented by an 'insured' to someone other than another 'insured'." If the tenant, or a guest of the tenant, takes your appliances, the sample does not pay.

Lost rent has a home in Coverage D. Fair Rental Value pays when a covered loss "makes that part of the 'residence premises' rented to others or held for rental by you not fit to live in," at "the fair rental value of such premises less any expenses that do not continue." It shares the single Coverage D limit with your own additional living expenses, so a fire that displaces both households draws both claims from one pot. It also excludes "loss or expense due to cancellation of a lease." Our additional living expense guide explains how that limit is set.

Liability usually survives, but read the condition. Section II excludes injury "arising out of or in connection with a 'business' conducted from an 'insured location'," then gives back "the rental or holding for rental of an 'insured location'" in three cases. One of them is renting "in part for use only as a residence, unless a single family unit is intended for use by the occupying family to lodge more than two roomers or boarders." A long-term residential tenant in part of your residence premises fits that give-back. The Massachusetts Division of Insurance still says a rented ADU "may need landlord coverage," and the two statements point different ways for a reason: the sample sets a floor, and an insurer's own form, endorsements and underwriting rules can be narrower. Your declarations page and endorsement list settle which applies to you.

The 10 percent cap on a detached ADU, worked in dollars

A detached unit that you or your family use rent-free is inside Coverage B. Whether Coverage B is big enough is the harder part. The sample says the Coverage B limit "will not be more than 10% of the limit of liability that applies to Coverage A," and that using it "does not reduce the Coverage A limit." One limit covers every other structure on the lot together. Every dollar figure in this table is an assumed input for one made-up property. None of them is rebuilding cost data, a benchmark or a quote.

Line H is the reason raising the dwelling limit is the wrong lever. To stretch a 10 percent cap over a $280,000 group of structures, the house would have to be insured for $2.8 million, and the premium for the house would rise with it. The better route is the one the Massachusetts Division of Insurance describes for a new structure: an endorsement that increases the other-structures limit or sets a separate limit for the unit. That keeps the dwelling limit tied to the house's own rebuilding cost, which is where it belongs. How that rebuilding cost is estimated is set out in how much dwelling coverage a home needs.

Why a partial loss is short too: the 80 percent test

A total loss is easy to picture. A kitchen fire in the cottage is more likely, and the sample's loss settlement condition can shrink that claim as well. Condition C.2 settles "buildings covered under Coverage A or B" at replacement cost only if "the amount of insurance in this policy on the damaged building is 80% or more of the full replacement cost of the building immediately before the loss." Below that line, the insurer pays the greater of the actual cash value of the damaged part, or "that proportion of the cost to repair or replace, after application of any deductible," which the insurance carried "bears to 80% of the replacement cost of the building."

The actual cash value floor matters here. For a brand-new unit, depreciation is small and actual cash value sits close to the repair cost, so the penalty is mild in the first years. As the cabinets, roof and flooring age, actual cash value falls and the 30 percent proportional figure starts to win, until the claim pays a fraction of the repair. The sample tests "the amount of insurance in this policy on the damaged building," and this example counts the whole Coverage B limit as that amount, which is the most generous reading. On any reading, a $60,000 limit on a $250,000 building fails the test by a wide margin, and the fix is the same separate limit that closes the total-loss gap.

One more detail: the garage and fence draw on the same $60,000. If a storm damages the garage roof for $12,000 in the same loss, only $48,000 is left for the cottage. A ground-mounted solar array or an in-ground pool shell adds to the same pool; see our pool and trampoline coverage guide for how those share the cap.

Attached units and the dwelling limit

An attached unit avoids the 10 percent cap, but it creates its own version of the same problem. The new square footage, kitchen and bathroom raise the house's rebuilding cost, and a dwelling limit left where it was before construction slides under the 80 percent line. Another illustration with stated inputs: a house with a rebuilding cost of $450,000 adds an attached unit costing $200,000 to rebuild, and the Coverage A limit stays at $450,000.

Raising the limit to at least $520,000 removes the partial-loss penalty, and raising it to the full $650,000 removes the total-loss gap. The Massachusetts Division of Insurance expects exactly this for conversions: when a basement or attached garage becomes a unit, the insurer "will likely deem it as a part of the primary structure and increase your dwelling coverage limits." The same logic applies to any renovation, and our guide to renovations that were never reported shows how the gap grows when the insurer never hears about the work.

A dwelling limit that rises also moves everything tied to it. In the sample, Coverage B is capped at 10 percent of A, and the Ordinance Or Law additional coverage lets you use "up to 10% of the limit of liability that applies to Coverage A" for code-driven costs. A percentage deductible grows too. Those are real increases in what the policy carries, not hidden fees, and our ordinance or law guide explains where the code allowance helps after a loss.

Massachusetts protected use ADUs: what the state permits and what its insurance regulator says

Massachusetts is a useful case because both its housing agency and its insurance regulator have published guidance on the same units. Governor Healey signed the Affordable Homes Act, Chapter 150 of the Acts of 2024, on August 6, 2024. According to the Executive Office of Housing and Livable Communities, its Section 8 amended the Zoning Act to allow ADUs up to 900 square feet to be built by right in single-family zoning districts, and inconsistent local zoning became unenforceable as of February 2, 2025. The agency's regulation, 760 CMR 71.00, took effect on January 31, 2025.

The agency's summary of a protected use ADU lists three requirements. The unit must keep a separate entrance, either directly from the outside or through a shared hall that meets the building code for safe egress. It must be no larger than half the gross floor area of the principal dwelling or 900 square feet, whichever is smaller. And it must meet local restrictions, which can include further size limits and limits on short-term rental. The state's page says ADUs can be inside an existing home, attached as an addition, or completely detached, like a backyard cottage or converted garage, so all three rows of the decision table above occur under the same law.

Two zoning rules have direct insurance consequences. First, cities and towns cannot "require owner occupancy nor family relationship for the ADU or the principal dwelling." The zoning law allows an owner to move out and rent both units, but the HO-3 sample's residence premises definition does not follow. The day the owner stops residing on the property, the homeowners form no longer describes it. Second, municipalities may restrict or prohibit short-term rentals, which the state's FAQ ties to stays subject to the room occupancy excise, meaning stays of not more than 31 consecutive calendar days. A unit rented monthly is outside that definition, and a unit rented by the night is inside it, which lines up closely with the split between this page and the short-term rental guide.

The Division of Insurance published its own question-and-answer notice on October 20, 2025. Its main points, in the order a homeowner meets them:

• Before construction: notify the insurer "before construction begins," and tell it if you intend to rent the unit, because renting "changes the risk characteristics of the property."
• Nonrenewal: an insurer may decline to renew when a property no longer meets its underwriting guidelines, but must send notice 45 days before the policy expires. Our nonrenewal guide covers what to do with that window.
• Type of coverage: a basement or attached-garage conversion is likely treated as part of the primary structure with a higher dwelling limit; a new structure likely needs "an endorsement to your policy to increase the limit or offer separate limits"; a rented unit may need landlord coverage including property damage, liability and loss of rental income.
• Premium: "Most likely, yes," the premium rises, because of higher replacement cost, added liability exposure and potential rental activity, with the size depending on the insurer, the unit's size and use, and the overall risk profile.
• If no standard insurer will write it: surplus lines coverage through a licensed broker, and the Massachusetts Property Insurance Underwriting Association, the state's FAIR Plan and insurer of last resort. Our FAIR Plan guide explains what that market covers.
• Tenants: the owner's policy "may include some coverage for tenants," but a tenant should consider renters insurance for personal possessions.

The regulator's answers line up with the HO-3 sample's structure: attached units go into the dwelling limit, detached units need a limit of their own, and a rental changes what the insurer is covering. They do not promise any outcome, and they could not, because each insurer writes its own forms and underwriting rules. Owners outside Massachusetts can use the same sequence, since the policy language behind it is the national ISO form, not a state rule.

Underwriting: why the insurer asks about the unit before it asks about the claim

A homeowners policy is priced on a description of the property: how many units, who lives there, what the building would cost to replace. An ADU changes all three. The sample's mortgage clause asks the lender to notify the insurer of "any change in ownership, occupancy or substantial change in risk" it is aware of, which shows how central occupancy is to the contract. And the sample's concealment or fraud condition voids coverage for an insured who has "intentionally concealed or misrepresented any material fact." An owner who rents a cottage for two years without mentioning it hands the insurer an argument that goes far beyond Coverage B.

The practical risk runs the other way too. Reporting the unit late, after a renewal questionnaire or an inspection reveals it, gives the insurer a choice it would not have had at the start: re-rate, add a separate dwelling policy, or decline to renew. Reporting it early, at the permit stage, gives you time to compare offers. The renewal shopping guide explains how to compare those offers on identical terms, with the unit already in the limits.

Construction itself has a short list of exposures. The sample covers "materials and supplies located on or next to the 'residence premises' used to construct, alter or repair the dwelling or other structures," so a fire that destroys stacked lumber is covered under the coverage the finished structure would fall under. Theft is the gap: the theft peril excludes loss "in or to a dwelling under construction, or of materials and supplies for use in the construction until the dwelling is finished and occupied." Copper, appliances waiting to be installed and tools left on site are the builder's or the builder's risk policy's concern during that window. On the vacancy side, the sample says "a dwelling being constructed is not considered vacant," which keeps the 60-day vacancy limits on vandalism and glass from applying to a site under active construction. Building a unit is one of several moments when a policy has to catch up with the house; the ownership and life events hub collects the others, from inheriting a house to selling one.

Five arrangements, side by side

The decision table above is clause by clause. Owners usually think in arrangements instead, so here are the five most common, with what each needs from the insurer.

The table's middle column is the useful one to bring to a phone call. Each item maps to one clause in the HO-3 sample, so a yes or no from the insurer can be checked against the endorsement list on the next declarations page. If the declarations page after the call still shows the old Coverage A figure and no endorsement for the unit, the conversation did not change the policy.

ADU insurance questions

Before the builder pulls a permit, pull your current declarations page and write the two numbers that matter next to your plans: the Coverage A limit and the Coverage B figure. If the unit will be detached, the gap between that Coverage B figure and the builder's cost to rebuild the unit is the separate limit to ask for, and our declarations page walkthrough shows where each figure is printed.