Pricing ranges
Addition and ADU projects use the Preliminary Estimate path. Three scope tiers anchor to two typical profiles — a ~500 sq ft mid-range addition (primary suite or family room) and a ~900 sq ft maximum by-right ADU under the Massachusetts Affordable Homes Act.
Anchor profile A — average home addition (~500 sq ft)
| Configuration A: primary suite | Configuration B: family room |
|---|---|
| Primary bedroom × 1 | Family room / great room × 1 |
| Full bathroom (en-suite) × 1 | Mudroom or office × 1 |
| Walk-in closet × 1 | — |
| Half bathroom × 0–1 | Half bathroom × 0–1 |
| 2–3 added spaces | 1–2 added spaces |
Anchor profile B — average ADU (~900 sq ft maximum by-right)
| Room / space | Count |
|---|---|
| Kitchen (full) | 1 |
| Full bathroom | 1 |
| Bedroom | 1–2 |
| Living room / great room | 1 |
| In-unit laundry | 1 |
| Closet / storage | 1–2 |
| Separate entrance | 1 |
| Typical rooms / spaces | 6–8 |
ADU configurations (all qualify under by-right law): detached ADU — new standalone structure (backyard cottage). Attached ADU — addition with separate entrance, connected to main home. Internal ADU — conversion of existing basement, attic, or garage into independent unit.
| Scope tier | Typical investment range | What's included |
|---|---|---|
| Basic scope | $145,000 – $245,000 | Small bump-out addition or interior conversion ADU. Minimal foundation, limited structural changes, smaller square footage |
| Mid-range scope | $245,000 – $345,000 | Mid-size addition (~500 sq ft) or attached ADU. Full foundation, framing, exterior envelope, systems extension or tie-in |
| Full-scope conversion | $345,000 – $445,000 | Large addition or detached ADU at maximum 900 sq ft. Full new construction with complete independent systems, exterior envelope, finish work |
ADU size caps by town
| Town | Cap |
|---|---|
| Newton | 1,000 sq ft by-right (up to 1,200-1,500 sq ft with special permit) |
| Brookline | 750 sq ft cap, smaller than the state baseline |
| Cambridge | State baseline (~900 sq ft) with minimal additional restrictions |
| Boston | Operates under its own zoning authority — state ADU framework does not apply |
Top five cost drivers
- Project type — detached ADU vs. attached addition vs. internal conversion. A detached ADU is effectively a small new home; internal conversion avoids foundation and exterior cost but adds plumbing, electrical, full kitchen build-out.
- Foundation work and Title 5 septic compliance — slab vs. frost wall vs. full basement; Title 5 upgrades $15K to $40K+ on private septic lots.
- Square footage, story count, and ADU size cap by town — 200 sq ft bump-out vs. 500 sq ft addition vs. 900 sq ft full ADU. Two-story costs more per sq ft than single-story.
- Systems — independent service vs. extension of existing. For ADUs, full independent systems significantly increase cost; for additions, panel upgrades, new HVAC zone, or expanded service shift cost.
- Architectural integration and historic district review — attached additions need to match existing architecture; detached ADUs have free-form exterior selections; historic districts (Brookline carriage house, Newton local historic, Cambridge historic, Boston Landmarks Commission) require independent approval.
Process narrative
Ten-step client-visible flow: Sales, Design & Planning, Procurement, Construction, and Completion. The same phase names appear on your proposal, your schedule, and on the job site.
- Consultation Scheduled (Sales Phase).
- Preliminary Proposal Sent & Preliminary Proposal Signed (Sales Phase).
- Site Measurement & Design Consultation (Design & Planning) — Validation Assessment for foundation, Title 5 septic, ADU zoning, historic district status.
- Material Selection Process (Design & Planning).
- Layout Development & Design Development & Presentation (Design & Planning).
- Final Proposal Sent, Negotiation & Contract Signed (Sales Phase).
- Design & Planning Summary Meeting + Design & Planning Completion Gate (Design & Planning).
- Permit Preparation & Submission (Design & Planning) — Long Form permit plus stamped engineered drawings; ADU permits 4 to 8 weeks in busier towns.
- Project Mobilization & Execution Phase (Construction).
- Final Completion Walkthrough & Project Completion (Completion).
Timeline by scope tier
| Scope tier | Typical duration | Includes |
|---|---|---|
| Basic scope | 6 to 8 months (design + permits + build) | Small bump-out addition or interior conversion ADU |
| Mid-range scope | 8 to 10 months (design + permits + build) | Mid-size addition or attached ADU — full foundation, framing, exterior envelope, systems extension or tie-in |
| Full-scope conversion | 10 to 12 months (design + permits + build) | Large addition or detached ADU at maximum 900 sq ft — full new construction with complete independent systems |
What's included (scope)
Selection categories scale 10-15 (basic) to 25-35+ (detached ADU at max size). Items unique to additions or ADUs versus interior packages:
- Foundation work (slab on grade / frost wall / full basement / underpinning at tie-in)
- Title 5 septic upgrade (+$15K to $40K+, private septic only)
- Structural tie-in to existing home (load-bearing modifications, second-story floor system)
- Full kitchen build-out (ADUs only — apartment-size refrigerator, single oven, stackable laundry common)
- In-unit laundry (compact stackable vs. full-size)
- Independent vs. shared utility services (subpanel, water, sewer, HVAC)
- Heat pump or mini-split systems for ADU HVAC
- Architectural integration package (matching siding profile, roof line, window style, trim) for attached additions
- Historic District Commission approval (Brookline carriage house, Newton local historic, Cambridge historic, Boston Landmarks)
Materials guidance
Top five selection-complexity drivers
- Whether the project includes a full kitchen (ADUs) — adds cabinets, hardware, countertops, tile (backsplash), plumbing fixtures, appliances.
- Whether the project includes a full bathroom — adds vanities, hardware, countertops, tile, plumbing fixtures, accessories, glazing & mirrors.
- Exterior envelope match (attached) or selection (detached) — match-existing for attached; net-new selection for detached.
- Architectural detail level — coffered ceilings, custom millwork, decorative shutters, porch columns, upgraded interior trim.
- Whether full independent systems require selections (ADUs) — heat pump model, mini-split units, exterior heat pump location, dedicated subpanel, laundry appliance.
Photo placement
Frequently asked
Why doesn't the additions and ADU guide have a fixed cost calculator?
Addition and ADU projects have too many scope variables that dramatically shift pricing — project type (addition vs. ADU), configuration (attached, detached, or interior conversion), foundation type, square footage, structural tie-in to existing home, and whether full systems are required.
What's the maximum ADU size I can build?
The state baseline is 900 sq ft by-right under the Massachusetts Affordable Homes Act, but local rules vary in our service area. Newton allows 1,000 sq ft by-right (and up to 1,200 to 1,500 sq ft with a special permit for certain configurations). Brookline caps at 750 sq ft, smaller than the state baseline. Cambridge follows the state baseline with minimal additional restrictions. Boston operates under its own zoning authority and the state ADU framework does not apply.
Will I need to upgrade my septic system?
For properties on private septic — common in Dover, Sherborn, Lincoln, Weston, and parts of Wellesley, Needham, and Lexington — Title 5 (310 CMR 15.000) is a critical cost driver. Massachusetts sizes septic systems by bedroom count. Adding a bedroom increases the property's total approved bedroom count, and if the existing system cannot handle the additional capacity, it must be upgraded before a building permit will be issued. Septic upgrades typically run $15,000 to $40,000+, and on constrained lots may not be physically possible. Properties on municipal sewer are generally unaffected.
How long do ADU permits take in Newton vs. Wellesley vs. Cambridge?
Additions and ADUs require Long Form permits with stamped engineered drawings, and review timelines vary significantly by town. Newton, Wellesley, Brookline, and Cambridge often have longer review cycles than smaller municipalities. ADU projects also fall under the Massachusetts Affordable Homes Act statewide framework — most municipalities have updated their bylaws since February 2025, but processing times for ADU permits can still take 4 to 8 weeks in busier towns.
What's the difference between an attached, detached, and internal ADU?
A detached ADU is a complete, standalone dwelling with its own foundation, full systems, kitchen, and bathroom — effectively a small new home. An attached ADU shares foundation tie-in and exterior envelope work with the principal home but uses extension of existing systems where possible. An internal ADU is a conversion of existing basement, attic, or garage to an independent dwelling unit — avoids foundation and exterior envelope cost but adds plumbing, electrical, and a full kitchen build-out within the existing structure.
My home is in a historic district. Can I still build an ADU?
Yes, but Historic District Commission approval is required and is independent of the by-right ADU framework. Districts include Brookline historic carriage house conversions, Newton local historic districts, Cambridge historic districts, and Boston Landmarks Commission. Historic review can require specific exterior materials, design details, and approvals that add design time and material cost.
Can my existing electrical panel handle an ADU?
Most ADUs require either an independent electrical service or a dedicated subpanel at minimum. For additions, whether existing HVAC, electrical, and plumbing capacity can absorb the new space — or whether a panel upgrade, new HVAC zone, or expanded service is required — shifts cost meaningfully.
Ready to put this into practice?
A 30-minute consult is usually enough to confirm whether we are the right fit.
Book a consult