ADU cost depends on the plan you choose, your lot conditions, and site work requirements. BuildX breaks every project into two clear parts — your home and your site work — so you always know exactly what you’re paying for.
A completed ADU home package ranges from $284,301 to $525,861 + Site Work. (Site Work is priced separately after a lot evaluation.)
Watch: John Perry walks through the Instant Estimate
BuildX breaks every project into two clear parts — your home and your site work. The plan price covers your complete home, foundation to finish: design, engineering, permitting, project management, and construction. Across our 14 floor plans it runs from $284,301 to $525,861. Site work — septic or sewer tie-in, water, electric, excavation, the driveway, tree clearing — depends on your lot, so we give you a firm number after a free lot review. Plan Price + Site Work = your final, all-in number.
The online estimator puts both parts into one planning range. It starts from the plan price, adds what each of your answers costs, adds your town’s permit and connection fees and two fixed soft costs, then widens the range for anything you are not sure about. Every figure below is generated from the same price list the estimator uses, so they always match it.
Amounts are what an answer adds compared with the standard choice. Finishes priced by the square foot are shown per sq ft and for a 900 sq ft plan; on a smaller plan they cost proportionally less. “Not sure” answers are not a choice with a price: the estimate carries the amount shown until it is confirmed, and your confidence score goes down.
What this means: The plan sets the size, the bedrooms and the starting price. Every answer after this adjusts from there.
Not sure yet? Pick the one closest to what you have in mind -- you can re-run this in 60 seconds with a different plan.
| Plan | Size | Plan price (building package) |
|---|---|---|
| X-1 The Cottage | 2 bed / 1.5 bath / 900 sq ft | $359,624 |
| X-2 The Hideaway | 1 bed / 1 bath / 610 sq ft | $284,301 |
| X-3 The Haven | 2 bed / 1 bath / 900 sq ft | $356,478 |
| X-4 The Atrium | 2 bed / 1.5 bath / 900 sq ft | $386,602 |
| X-5 The Glen | 2 bed / 1 bath / 890 sq ft | $358,700 |
| X-6 The Nest | 1 bed / 1.5 bath / 900 sq ft | $488,600 |
| X-7 The Gables | 2 bed / 1 bath / 900 sq ft | $384,600 |
| X-8 The Garden House | 2 bed / 1.5 bath / 900 sq ft | $346,502 |
| X-9 The Breeze | 2 bed / 2 bath / 900 sq ft | $372,065 |
| X-10 The Nook | 1 bed / 1 bath / 667 sq ft | $314,602 |
| X-11 The Meadow | 1 bed / 1 bath / 775 sq ft | $345,505 |
| X-12 The Homestead | 2 bed / 2 bath / 900 sq ft | $525,861 |
| X-14 The Woodland | 1 bed / 1.5 bath / 725 sq ft | $332,500 |
| X-15 The Retreat | 1 bed / 1 bath / 900 sq ft | $405,062 |
What this means: Detached is its own building in your yard. Attached is connected to your house. Attached vs detached
| Answer | What it adds |
|---|---|
| Detached -- its own standalone building | Included as standard |
| Attached -- connected to the existing houseAttached units need structural tie-in, roof and siding matching, and a fire separation assembly. It is almost never cheaper than detached. | + about $5,000 - $10,000 |
| I'm not sure yet | Carries $0 - $10,000 until confirmed |
This doesn't change your price. It changes what we'd recommend.
| Answer | What it adds |
|---|---|
| An aging parent or relative | No added cost |
| An adult child | No added cost |
| Rental income | No added cost |
| Guests / flex space | No added cost |
| Home office or studio | No added cost |
| Property value / future flexibility | No added cost |
| Not sure yet | No added cost |
Thanks for taking the time. Because permit fees, water and sewer connection charges, and local requirements are different in every single town, telling us where you're building is the fastest way to a number you can actually trust.
Your town does not add a flat amount. It sets the fees below, which the estimator adds from its town table:
| Fee | When it is added | In our towns | Outside our service area |
|---|---|---|---|
| Building permit | Always | $1,000 - $2,500 | $1,000 - $4,000 |
| Water connection fee | If you answer town water | $1,500 - $5,000 | $1,500 - $5,000 |
| Sewer connection / betterment | If you answer town sewer | $5,000 - $10,000 | $5,000 - $10,000 |
We build in 40 Massachusetts towns. South Shore: Abington, Avon, Brockton, Cohasset, Duxbury, East Bridgewater, Hanover, Hanson, Hingham, Holbrook, Kingston, Marshfield, Norwell, Pembroke, Plymouth, Plympton, Rockland, Scituate, Stoughton, West Bridgewater, Weymouth, Whitman. South Coast: Acushnet, Fairhaven, Freetown, Lakeville, Marion, Mattapoisett, Middleborough, Rochester, Wareham. Upper Cape Cod: Bourne, Falmouth, Mashpee, Sandwich. Bridgewater Area: Bridgewater, Carver, Easton, Halifax, Raynham.
What this means: The outside skin of the ADU. Vinyl is included. The others are priced by the square foot, so they cost more on a bigger plan.
| Answer | What it adds (900 sq ft plan) | Per sq ft |
|---|---|---|
| Vinyl siding | Included as standard | Included |
| Fiber cement (HardiePlank) | + about $6,300 - $9,000 | $7 - $10 |
| Cedar shingles | + about $9,900 - $16,200 | $11 - $18 |
| Wood clapboard | + about $12,600 - $18,000 | $14 - $20 |
| Not sure yet | Carries $0 - $9,000 until confirmed | $0 - $10 |
What this means: Architectural asphalt shingle is included. Standing seam metal is priced by the square foot of your plan.
| Answer | What it adds (900 sq ft plan) | Per sq ft |
|---|---|---|
| Architectural asphalt shingle | Included as standard | Included |
| Standing seam metal | + about $16,200 - $22,500 | $18 - $25 |
| Not sure yet | Carries $0 - $9,000 until confirmed | $0 - $10 |
What this means: The color of the window frames on the outside. White is usually the cheapest. Black or bronze costs more.
| Answer | What it adds |
|---|---|
| White | Included as standard |
| Black or bronze exterior | + about $4,800 - $8,500 |
| Not sure yet | Carries $0 - $5,000 until confirmed |
What this means: Luxury vinyl plank (LVP) is included. Wood and tile are priced by the square foot of your plan.
| Answer | What it adds (900 sq ft plan) | Per sq ft |
|---|---|---|
| Luxury vinyl plank (LVP) | Included as standard | Included |
| Engineered hardwood | + about $4,500 - $8,100 | $5 - $9 |
| Site-finished solid hardwood | + about $5,400 - $8,100 | $6 - $9 |
| Tile throughout | + about $8,100 - $10,800 | $9 - $12 |
| Something else / not sure | + about $0 - $3,600 | $0 - $4 |
What this means: The finish on your interior trim and doors. Painted is included. Stained natural wood costs more.
| Answer | What it adds |
|---|---|
| Painted | Included as standard |
| Stained natural wood | + about $6,500 - $14,000 |
| Not sure yet | Carries $0 - $5,000 until confirmed |
What this means: Painted cabinets are included. Stained wood costs more.
| Answer | What it adds |
|---|---|
| Painted | Included as standard |
| Stained wood | + about $3,500 - $9,000 |
| Not sure yet | Carries $0 - $5,000 until confirmed |
What this means: Quartz is included. Granite and premium stone or slab can cost more.
| Answer | What it adds |
|---|---|
| Quartz | Included as standard |
| Granite | + about $0 - $3,200 |
| Premium stone / porcelain slab / other | + about $3,500 - $11,000 |
| Not sure yet | Carries $0 - $11,000 until confirmed |
What this means: How the ADU is heated and cooled. The standard is an all-electric heat pump that does both. Gas or propane adds forced hot air plus a separate A/C. Ducted vs ductless
| Answer | What it adds |
|---|---|
| All-electric heat pump (heat + A/C) | Included as standard |
| Fully ducted heat pump system | + about $1,000 |
| Gas or propane forced hot air + separate A/C | + about $9,500 - $19,000 |
| Not sure -- recommend something | Carries $0 - $9,000 until confirmed |
What this means: how power reaches your property today shapes what it takes to run the ADU its own service.
Every BuildX ADU gets its own meter. Sharing the existing house meter is not something we do.
| Answer | What it adds |
|---|---|
| Overhead service (wires on poles) | + about $2,500 - $6,500 |
| Underground service | + about $2,000 - $6,500 |
| Not sure | Carries $1,500 - $6,000 until confirmed |
What this means: gas or propane means running a line or setting a tank. All-electric needs neither and is the standard.
| Answer | What it adds |
|---|---|
| Natural gas is at the street / already at my house | + about $1,500 - $5,500 |
| Propane -- I have or would add a tank | + about $2,200 - $4,000 |
| All-electric, no gas | Included as standard |
| Not sure | Carries $0 - $4,000 until confirmed |
Slope drives excavation, retaining, and drainage -- three of the most expensive site items.
| Answer | What it adds |
|---|---|
| Flat | + about $0 - $3,500 |
| Gently sloped | + about $4,500 - $14,000 |
| Steeply sloped or very uneven | + about $10,000 - $28,000 |
| I'm not sure | Carries $2,000 - $22,000 until confirmed |
What this means: trees in the build area have to be cleared before work starts.
| Answer | What it adds |
|---|---|
| Open, no clearing needed | No added cost |
| A few trees to remove (1-5) | + about $2,500 - $4,000 |
| Heavily wooded (6+ trees) | + about $4,000 - $8,000 |
| Not sure | Carries $1,000 - $7,500 until confirmed |
What this means: Ledge is solid rock or large boulders in the ground where the ADU would sit.
This is the #1 surprise cost on South Shore lots. We'd rather show you a wide range now than a change order later.
| Answer | What it adds |
|---|---|
| No known ledge | + about $0 - $1,500 |
| Possibly -- neighbors have hit it, or there's exposed rock | + about $6,000 - $28,000 |
| Yes, confirmed ledge | + about $18,000 - $30,000 |
| No idea | Carries $0 - $5,000 until confirmed |
What this means: anything in the way has to be taken down or moved before we can build.
| Answer | What it adds |
|---|---|
| Nothing | No added cost |
| A shed or small outbuilding | + about $1,200 - $4,500 |
| A garage or large structure | + about $6,000 - $12,000 |
| A pool | + about $9,000 - $12,000 |
| Deck, patio, or hardscape | + about $2,000 - $5,500 |
| More than one of these, or something else | + about $2,000 - $15,000 |
What this means: Whether the ADU can connect to your existing driveway or needs a new one.
| Answer | What it adds |
|---|---|
| Use the existing driveway as-is | Included as standard |
| Extend or widen the existing driveway | + about $4,500 - $8,000 |
| Build a new driveway from the street | + about $10,000 - $15,000 |
| Not sure | Carries $0 - $5,000 until confirmed |
What this means: town water means a connection fee set by your town. A private well may need a flow test, or a new well for the ADU.
| Answer | What it adds |
|---|---|
| Town / city waterConnection fee added from your town. | No flat amount (see note) |
| Private well -- tie the ADU into the existing wellRequires a flow test. If your well can't carry two dwellings, this changes. | + about $3,500 - $6,000 |
| Private well -- drill a new well for the ADU | + about $6,000 - $12,000 |
| Not sure | Carries $2,000 - $12,000 until confirmed |
What this means: septic is the biggest cost variable in an ADU. Town sewer means a connection fee instead. Why septic matters
| Answer | What it adds |
|---|---|
| Town sewerConnection / betterment fee added from your town. | No flat amount (see note) |
| Private septic systemA few more questions coming -- this is the big one. | No flat amount (see note) |
| I'm not sure | Carries $6,000 - $25,000 until confirmed |
Under Massachusetts Title 5, your house and your ADU are treated as one facility. Adding bedrooms can push your system over capacity and force a full upgrade -- this is the single most common reason an ADU budget moves.
| Answer | What it adds |
|---|---|
| Replaced within the last 5 years and passed Title 5 | + about $3,000 - $9,000 |
| Passed Title 5 in the last 5-15 years | + about $6,000 - $19,000 |
| Older than 15 years, or I don't know its condition | + about $16,000 - $28,000 |
| Known to be failing, or it's a cesspool | + about $22,000 - $38,000 |
| I have no idea | Carries $6,000 - $30,000 until confirmed |
What this means: if an engineer has already designed a plan or done a soil test, part of that design work is done.
| Answer | What it adds |
|---|---|
| Yes, we have a plan or a soil test | Saves about $0 - $3,000 |
| No | + about $3,500 - $6,000 |
| Not sure | Carries $1,500 - $8,000 until confirmed |
What this means: The 44-inch crawl space is included. Each other choice shows what it adds. Foundation types
* Not every town allows a full basement under an ADU, and a finished basement can count against your 900 sq ft cap. We confirm this with your building department before design.
| Answer | What it adds |
|---|---|
| 44" crawl space | Included as standard |
| 6'7" tall crawl space -- usable storage | + about $11,000 - $16,000 |
| Full-height basement * | + about $19,000 - $28,000 |
| Slab on grade | + about $5,000 - $8,500 |
| Not sure | Carries $0 - $30,000 until confirmed |
Site and utility answers are where most of the spread in an ADU budget comes from. Using the same figures as above:
| Score | Label | Contingency added | What the estimator tells you |
|---|---|---|---|
| 0% to 30% | Very low confidence | 15% | You told us you're unsure about most of your property, so we've priced the risk honestly rather than quoting you a number we'd have to walk back. A site visit would change this range dramatically. |
| Over 30% to 50% | Low confidence | 13% | There are enough unknowns here that this is a starting point, not a budget. The good news: none of them are expensive to find out. |
| Over 50% to 70% | Moderate confidence | 10% | A handful of unknowns are widening this range. Every one of them is answerable in a single site visit. |
| Over 70% to 85% | Good confidence | 7% | This is a solid working budget. A free site evaluation would tighten it by roughly half. |
| Over 85% to 100% | High confidence | 5% | You clearly know your property. This is about as accurate as a number gets without us standing on your lot. |
This is a planning range built from your answers, not a quote. Site conditions, town requirements and your final selections all move the number. A site visit typically tightens this range by about half. Get your estimate with the tool above
Six of our most popular floor plans, with starting prices. Each price includes the complete building package — foundation through move-in-ready finishes. Tap any plan to see the full floor plan and details.
The Hideaway delivers 610 GFA in a compact 20'-30' footprint, with 1 bedroom, 1 full bath, and a Covered Entryway, plus 9' ceilings, LED lighting, solar-ready infrastructure, and ADA-ready options.
View this plan →
Most Popular
The Haven delivers 900 GFA in a compact 28'-36' footprint, with 2 bedrooms, 1 bath, and a Covered Entryway, plus 9' ceilings, LED lighting, solar-ready infrastructure, and ADA-ready options.
View this plan →
The Atrium delivers 900 GFA with 2 bedrooms, 1.5 bath, and a Covered Entryway, plus 9' ceilings, LED lighting, solar-ready infrastructure, and ADA-ready options.
View this plan →
The Gables delivers 900 GFA with 2 bedrooms, 1 bath, and a 2-Car Garage, plus 9' ceilings, LED lighting, solar-ready infrastructure, and ADA-ready options.
View this plan →
The Breeze delivers 900 GFA in a compact 30'-36' footprint, with 2 bedrooms, 2 baths, and a Farmer's Porch, plus 9' ceilings, LED lighting, solar-ready infrastructure, and ADA-ready options.
View this plan →
The Woodland delivers 725 GFA in a compact 26'-36' footprint, with 1 bedroom, 1.5 bath, a Farmer's Porch, plus 10' ceilings, LED lighting, solar-ready infrastructure, and ADA-ready options.
View this plan →* Prices include the complete building package: foundation, framing, roof, siding, windows, electrical, plumbing, HVAC, kitchen, bathroom, flooring, trim, paint, permits, and project management. Site work is priced separately after a lot evaluation.
View all 14 floor plans →Every ADU project is different because every lot is different. These are real BuildX families, real pricing, and real explanations of what drove the cost.
Modified X-5 The Glen
A modified X-5 Glen built in Marshfield, custom-engineered with a whole-house ERV system, zero adhesives, and low-VOC materials to create a healthier living environment for a family member with an autoimmune condition.
Modified X-2 The Hideaway
Recently toured by Lt. Governor Kim Driscoll, a modified X-2 Hideaway was built in Plymouth, bringing a loved one closer to home for better care, comfort, and peace of mind.
X-3 The Haven
An X-3 Haven with a walk-out basement built in Kingston, designed to support multigenerational living by helping family members remain close while enjoying their own space.
X-3 The Haven
An X-3 Haven built in Freetown, providing a flexible housing solution that helped a family create an affordable path to homeownership for the next generation. Features include a full basement, expanded deck, and blue shaker cabinets.
Custom In-Law Addition
A custom in-law addition in Pembroke, connected to the main home by a 7-foot breezeway and designed to provide independent living just steps away from family support.
Not all ADU pricing is apples-to-apples. Understanding what’s included — and what’s not — is the difference between an accurate budget and an expensive surprise.
The cheapest ADU quote is often the most expensive project. Modular “starting at” prices don’t include the site work, permitting, utility connections, and finish work that can double the final cost. When comparing quotes, compare total project scope — not just the sticker price.
Modular vs. site-built ADUs — a real cost comparison for Massachusetts homeowners.
The price you see on our website covers your home — everything from the foundation up. Site work — preparing your land — varies by property, so we price it separately after evaluating your lot. No surprises mid-project.
That is the full span of our 14 floor plans. Prices include the complete building package, foundation through move-in-ready finishes. Site work is priced separately after a free lot review, so your final price is always Plan Price + Site Work.
Your complete home, foundation to finish: design, engineering, permitting, project management, and construction. Once you sign, it does not change.
Only what your lot needs: septic or sewer tie-in, water, electric, excavation, the driveway, tree clearing. Carried at a worst-case allowance, then credited back if unused.
That’s the whole formula. No surprise change orders, and we’re never financially ahead of you. We give you a firm number only after we’ve seen your lot, at no cost, because a low number that climbs later isn’t a real number.
These aren’t hidden fees. They’re real project costs that many homeowners don’t think about until they’re in the middle of a project. We surface them upfront so you can budget accurately.
Flexible payment options available for qualified buyers. Ask about financing during your call with our team.
The questions our families ask most about ADU costs in Massachusetts.
BuildX ADUs range from $284,301 for our 610 sq ft Hideaway floor plan to $525,861 for our 900 sq ft Homestead floor plan. These prices include the complete home package — foundation through move-in-ready finishes. Site work is priced separately after a lot evaluation and typically ranges from $50,000–$150,000+ depending on lot conditions. Read the full breakdown of what drives ADU cost in Massachusetts.
Scope differences. A lower quote may exclude site work, permitting, utility connections, or finish work. “Starting at” prices for modular or prefab ADUs rarely include the full cost of getting the unit on your lot and connected to utilities. Always compare total project scope — not just the sticker price.
Everything from the foundation up: framing, roof, siding, windows, electrical, plumbing, HVAC, kitchen, bathroom, flooring, trim, paint, permits, and project management. A move-in-ready ADU.
Site work — tree clearing, excavation, grading, septic, water line, electric line, driveway, drainage, and retaining walls. We handle all of it, but we price it after evaluating your specific lot because every property is different. You get a fixed site work price before any construction begins.
It depends entirely on your lot. A flat, cleared lot with sewer access might be on the lower end ($50,000–$75,000). A wooded lot with ledge, wetlands, and a septic requirement could be significantly more ($80,000–$105,000). We give you a fixed site work price before breaking ground — no surprises mid-project.
The sticker price looks lower, but the total project cost often isn’t. Modular “starting at” prices typically exclude delivery, crane, foundation, site prep, utility connections, permitting, and finish work. Those costs can add $100,000+ to the final number. A custom-built ADU from BuildX includes the full home package in the quoted price.
Yes. We have financing options available for qualified buyers. Ask about terms during your call with our team.
Our target is 100 days from contract to completion for the construction phase. Permitting timelines vary by town and can add 90-120+ days before construction starts. We handle all permitting.
Yes, within structural limits. Many families make modifications to our standard plans — adjusting layouts, adding features, or upgrading finishes. Customization may affect pricing. We walk through all options during the design phase.
Schedule a call. We’ll discuss your goals, review your property, and give you real numbers — no guessing, no pressure. You can also book a private showing to walk through a finished unit in person before making any decisions.
Let’s take a look at your property and give you real numbers — no guessing, no pressure.
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