Boston · Suffolk County · MA Licensed Bathroom Pro

ADU Builders in Boston, MA.

Full granny-flat installation for Boston, MA homes. Detached new construction, garage conversion, basement conversion, or attached ADU. Massachusetts HOMES Act 2025 compliant. 4 to 9 month timeline. $100K–$350K typical investment. Run under unified coordinator from zoning research through certificate of occupancy.

ADU construction in Boston requires two specialized capabilities most contractors lack: HOMES Act compliance research (the new MA state framework that overrides many local zoning restrictions for ADUs under 900 sqft) and full miniature-home construction (ADUs are complete dwellings — kitchen, bath, sleeping area, separate utilities). Our crew carry each required Massachusetts trade license directly — Construction Supervisor, Master Plumber, Master Electrician, Master Gas Fitter, EPA RRP. Single project manager owns zoning compliance check, design, permit pull, foundation, framing, envelope, all interior trades, and final inspection across the Boston ADU project.

Why ADU Projects in Boston Are Specialized Work

ADU projects in Boston run at the junction of a few factors unique to accessory dwelling units:

  1. HOMES Act compliance + local zoning interaction. The Massachusetts HOMES Act (2025) makes ADUs by-right on single-family-zoned lots up to 900 sqft, overriding many local zoning prohibitions. However, dimensional limits (setbacks, height, lot coverage) and design standards still apply per local bylaw. We run zoning compliance research at the design consultation to confirm the project fits within HOMES Act + Boston bylaw before any drawings are commissioned.
  2. Full miniature-home construction. An ADU is a complete dwelling unit — full kitchen with appliances, full bathroom, sleeping area or bedrooms, separate utilities (electrical sub-panel from main, separate plumbing supply/drain), HVAC system. The typical Boston ADU build engages 10 to 12 trades in concentrated form within a 600-900 sqft footprint.
  3. Owner-occupancy and rental-use considerations. Many Boston bylaws under HOMES Act compliance retain owner-occupancy requirements (the property owner must reside in either the primary dwelling or the ADU). Short-term rental restrictions vary by town. Pro Build flags these compliance requirements during design consultation to align with intended use.

Boston Metro Core ADU projects face the highest demand and tightest constraints in MA: small urban lots (3,000-6,000 sqft) where rear-yard ADU placement consumes most or all available outdoor space. Most Boston-area ADUs are basement conversions or attached additions rather than detached new builds. The MA HOMES Act (2025) overrides many local zoning restrictions, but design must still respect setbacks, max height, and lot coverage limits.

Service area: Boston, Massachusetts. View larger map

Free Bathroom Design Consult for Your Boston Home

On-site visit + 3D layout + line-item written estimate within 5 days. No commitment.

What Is the Investment Range for Boston ADUs

Project pricing varies based on ADU type (detached vs garage conversion vs basement), foundation requirements, finish level, and utility connection complexity. Standard tiers for Boston ADU builds:

ADU Type Sq Ft Investment Timeline
Garage conversion (existing structure)400–600$95,000–$170,0003–5 months
Basement conversion (walkout preferred)500–900$120,000–$220,0003–5 months
Detached new construction (mid-spec)600–800$220,000–$320,0005–8 months
Detached premium (architect-design, custom)800–900$320,000–$450,0007–10 months

Boston approval outlays for ADU projects run $200–$950+ on standard scopes. Septic-system upgrade (when required for additional bedroom) adds $15,000-$30,000. Utility connection fees vary by municipality and existing service capacity.

Boston's metropolitan-scale market combined with the new MA HOMES Act creates the highest ADU demand in the Commonwealth. Pro Build runs multiple ADU projects concurrently in the area. Permit turnaround at the Boston building department averages 4–10 weeks as municipalities adapt to HOMES Act compliance frameworks.

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Construction Specs for Boston ADUs

Specification specification for Boston ADU builds reflects the dual demands of compact construction and long-term rental durability.

Foundation
Detached ADUs: slab-on-grade ($15-$25/sqft) most common for cost; crawlspace ($25-$40/sqft) for moisture-prone sites. Frost-line depth 48" in MA Zone 5A drives footing depth.
Structural framing
2x6 exterior wall framing (24" OC for energy efficiency), engineered LVL beams for kitchen-area open spans. Roof: pre-engineered trusses standard for cost-efficient detached ADU. Hurricane straps at all framing connections.
Envelope
Siding: James Hardie fiber-cement (most popular), LP SmartSide for budget. Windows: Andersen 100 series or Pella Encompass for mid-tier energy-efficiency. Triple-pane upgrade for premium tier (energy savings on rental ADU pays back in 5-7 years).
Interior + MEP (mechanical, electrical, plumbing)
Heat pump mini-split (Mitsubishi M-Series or Fujitsu Halcyon, 1-2 ton single-zone for under 900 sqft). Tankless or 30-40 gallon electric water heater. Sub-panel from main service (typically 100-amp). Stackable washer/dryer in bath if rental-use intended.

The appropriate selection for the Boston ADU depends intended use (family vs rental) and budget. Free design consultation produces written specification within 5-7 days.

Window, Door, and Appliance Brands We Install for Boston ADUs

ADU brand selection balances long-term rental durability against budget. Pro Build sources only manufacturer-authorized lines through factory distribution.

  • Andersen 100 / 200 / 400 windows (rental-grade durability)
  • Pella Encompass / Architect windows
  • James Hardie fiber-cement siding (lifetime warranty for rental)
  • LP SmartSide trim and accent siding
  • Mitsubishi M-Series + Fujitsu Halcyon mini-split heat pumps
  • Rinnai tankless water heaters (gas or electric)
  • Bosch 500 series compact appliances (kitchen package)
  • Kohler + Moen bath and kitchen fixtures

Brand selection for the Boston ADU gets documented on the written estimate with model numbers, finish codes, and warranty terms detailed line by line. Compact-spec appliances (24" depth refrigeration, 18" dishwasher) chosen for kitchen footprint efficiency.

How Boston ADU Permits Work Under the HOMES Act

Each ADU builds in Boston demand a 780 CMR building permit + foundation, framing, electrical, plumbing, mechanical sub-permits + a Massachusetts HOMES Act compliance application with the Boston planning department. Pre-1978 home conversions (basement or garage) call for EPA RRP lead-safe procedures.

Pro Build submit every approval under our Massachusetts CSL, coordinate all required inspections with the Boston Inspectional Services Department, and navigate the new HOMES Act compliance pathway with the Boston planning office. The homeowner signs nothing at the building department or planning office.

The Pro Build ADU Workflow for Boston

Every Pro Build Boston ADU build follows the same six-step workflow.

  1. Free on-site consultation + HOMES Act zoning research. Lead Construction Supervisor walks the property, measures existing structures, and confirms HOMES Act + local bylaw compliance pathway before any design work. Total: 1-2 weeks.
  2. Design + structural engineering. ADU layout (kitchen, bath, sleeping area), site plan, foundation plan, MEP coordination. Total: 4-8 weeks.
  3. Permit filing. Building, foundation, electrical, plumbing, mechanical sub-permits + utility coordination filings. ADU-specific applications under the HOMES Act framework with Boston planning department.
  4. Foundation + framing. Excavation, footings, foundation walls (slab, crawlspace, or full basement), then framing and roof — for detached. For conversions: structural reinforcement, wall additions, plumbing/electrical reconfiguration.
  5. MEP rough-in + envelope. Electrical sub-panel from main service, plumbing supply/drain, HVAC (typically mini-split heat pump for energy efficiency), insulation, drywall, exterior siding/windows.
  6. Interior finish + utility activation + final inspection. Kitchen install, bath install, flooring, trim, paint, fixture install, separate utility metering activation (where required), then certificate of occupancy from the Boston Inspectional Services Department.

Six steps, every Boston ADU build. Single project manager owns the work from first consultation through certificate of occupancy across the 4-9 month project window.

DOE Climate Zone 5A — covering most of Massachusetts including Boston — drives ADU envelope specs: minimum R-21 wall insulation, R-49 attic insulation, U-0.30 windows. ADU mechanical sizing typically heat-pump mini-split (1.5-2 ton single-zone for under 900 sqft) for energy efficiency.

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Six Things to Verify Before Hiring a Boston ADU Contractor

ADU contractor selection requires verifying capabilities most general contractors lack — the HOMES Act framework is new and most contractors have not yet built ADUs under the 2025 rules. Verify all of the following before signing anything for a Boston ADU build:

MA Construction Supervisor License (CSL) — UNRESTRICTED grade
ADU detached construction qualifies as a separate dwelling and requires UNRESTRICTED CSL.
Demonstrated HOMES Act ADU experience
The MA HOMES Act took effect in 2025. Verify the contractor has completed at least 2-3 ADU projects under the new framework with successful permit approvals.
In-house Master Plumber + Master Electrician licenses
ADU MEP work is concentrated and complex (full kitchen, full bath, separate utility coordination). Subbed MEP creates coordination gaps and warranty disputes.
Documented relationships with structural engineer + civil engineer (when site work needed)
Detached ADUs on uneven terrain or expansive sites may require civil engineer for stormwater/site grading. Established relationships save 4-8 weeks vs ad-hoc engagement.
HIC Registration + Insurance (Builder's Risk on detached ADU)
Builder's Risk insurance covers the partially-completed ADU during construction.
MA HIC arbitration eligibility
HIC-registered contractors are bound by the MA HIC arbitration program — homeowner protection of last resort.

Pro Build meets all six criteria on every Boston ADU build. Verify credentials directly via the Massachusetts Office of Consumer Affairs HIC database before signing any contract.

Project Spotlight: Boston Accessory Dwelling Unit

What Warranty Comes With a Boston ADU

Every Pro Build Boston ADU build is backed by multiple layers of coverage:

Window and door manufacturer warranties
Andersen 100/200: 20-year glass / 10-year sash. Pella Encompass: 10-year limited. Registered on the homeowner behalf within 14 days.
Siding manufacturer warranty
James Hardie: 30-year limited transferable. LP SmartSide: 50-year limited.
HVAC mini-split warranty
Mitsubishi M-Series: 12-year compressor / 7-year parts. Fujitsu Halcyon: similar terms.
Pro Build workmanship guarantee
5 years on installation labor including foundation, framing, envelope, all interior trades. We return at no cost if anything we installed needs adjustment within the warranty period.
Massachusetts code + HOMES Act compliance
All work passes 780 CMR Massachusetts code inspection + HOMES Act compliance verification. Final certificate of occupancy issued by Boston Inspectional Services Department.
Massachusetts HIC compliance
Every project governed by an MA HIC-compliant written contract per MGL c. 142A.

The six layers of backing compound so the Boston owner carries protection on every failure mode for the long-term life of the ADU.

What Sets Pro Build Apart for Boston ADU Builds

Most Boston contractors have not yet completed ADU projects under the new MA HOMES Act 2025 framework. Pro Build has built ADUs under the new state law since the rollout — including detached new construction, garage conversions, and basement conversions across the Commonwealth.

  • HOMES Act fluency. We track every state guidance update and run zoning compliance research before any design begins.
  • One project, one PM, one warranty. Multi-month coordination under one contract — no homeowner managing 8 contractors.
  • In-house MEP licenses. Master Plumber, Master Electrician, Master Gas Fitter — no subcontractor handoffs across the concentrated ADU MEP scope.
  • Compact-construction expertise. Specifying 24" depth appliances, stackable laundry, and dual-purpose furniture-built-in spaces requires different design discipline than full-house construction.
  • 5-min response during business hours. Faster than industry standard. We pick up.
  • Boston-area design crew. Local zoning bylaw fluency saves weeks on every ADU project.

These six reasons reinforce on a unified ADU project. The HOMES Act fluency alone is the difference between a 4-week permit pathway and a 4-month permit pathway.

Cross-Trade Coordination on Boston ADU Builds

Every Boston ADU build engages the full set of residential trades concentrated in a small footprint. Pro Build holds every relevant license in-house:

  • Excavation + foundation — site survey, footing/foundation work, slab pour, drainage. Detached ADUs typically slab-on-grade for cost efficiency.
  • Framing + structural — wall framing per engineered drawings, roof framing (typically pre-engineered trusses).
  • Envelope — siding install, window/door install, roof shingles, exterior trim.
  • Electrical (with sub-panel from main service) — sub-panel install, full circuiting for ADU kitchen + bath + bedroom + HVAC + appliances.
  • Plumbing (with separate supply/drain to main service) — full bath rough-in, kitchen rough-in, water heater install, sewer/septic tie-in.
  • HVAC (mini-split most common) — heat-pump install, refrigerant lines, electrical tie-in.
  • Insulation + air sealing — Zone 5A or 6A code minimums, energy performance verified.
  • Interior finish — drywall, paint, trim, flooring, kitchen install, bath install, fixture trim.

Every related item stays coordinated under the single project manager and the unified written contract.

Neighborhoods Covered for Boston ADU Builds

Every Boston ZIP gets the same crew, same project manager, and same 5-year workmanship guarantee.

ADU Services in Cities Adjacent to Boston

Pro Build crews work Boston and the surrounding Suffolk County markets daily. Same crew familiarity transfers across municipal lines.

Customer Stories

Real Boston ADU Builders Stories.

  • ★★★★★
    "Pro Build delivered exactly what they quoted on our Boston bath remodel. Single PM, daily updates, on-budget. Better than any contractor we have hired."

    Sarah M.

    Boston, MA · Verified Google Review
  • ★★★★★
    "Got three quotes for our Boston master bath. Pro Build was the only one with vanity elevations and a line-item estimate within 5 days. Hired them."

    David R.

    Boston, MA · Verified Google Review
  • ★★★★★
    "Cross-trade coordination sold me. They handled vanity, tile, plumbing, electrical, and venting under one project manager. Saved me weeks of coordination."

    Jennifer T.

    Boston, MA · Verified Google Review

Boston ADU Builders FAQs

Boston ADU Builders Questions Answered.

What is the MA HOMES Act and how does it affect ADUs in Boston?

The Massachusetts HOMES Act (effective 2025) makes ADUs by-right on single-family-zoned lots across the state — overriding many local zoning prohibitions. ADUs up to 900 sqft can be permitted without special permit or variance. Owner-occupancy requirements and dimensional standards (setbacks, height, lot coverage) still apply per local bylaw. Pro Build runs HOMES Act + Boston bylaw compliance research at the design consultation.

What duration applies to Boston ADU builds?

Garage or basement conversions: 3 to 5 months from permit to certificate of occupancy. Detached new construction: 5 to 9 months. Design + permit phase adds 2 to 4 months before construction starts.

Are ADU rental rules different in Boston?

Long-term rental (30+ days) generally permitted for ADUs in Boston under the HOMES Act framework. Short-term rental rules (under 30 days, e.g. Airbnb/VRBO) vary by municipality — many MA towns prohibit short-term rental in ADUs even when long-term is permitted. Pro Build flags rental-use compliance during design consultation.

Can I build an ADU on a non-owner-occupied property in Boston?

Many Boston bylaws under HOMES Act compliance retain owner-occupancy requirements (the property owner must reside in either the primary dwelling or the ADU — not both rented to others). Specific rule varies by municipality. Pro Build verifies the Boston owner-occupancy requirement during the zoning research phase.

Are there size limits for ADUs in Boston?

Under the MA HOMES Act, ADUs up to 900 sqft qualify for by-right permitting in single-family zones. Larger ADUs may be permitted but require special permit / zoning compliance pathway under Boston bylaw. Detached, attached, garage-conversion, and basement-conversion ADUs all eligible.

What about wells and septic for Boston ADUs?

Properties on municipal sewer: typically no upgrade needed for ADU. Properties on private septic: existing septic must be evaluated for additional bedroom capacity per Title 5 requirements. Septic upgrade (when required) adds $15,000-$30,000 and 2-4 months to project timeline.

How small a lot can have an ADU in Boston?

The MA HOMES Act removes most lot-size minimums for ADU construction in single-family zones. Boston bylaw setback and lot-coverage limits still apply, which constrains ADU placement on small lots. Pro Build runs zoning compliance research at the design consultation to confirm feasibility.

How are utilities handled on a Boston ADU?

Detached ADU electrical typically wired to a sub-panel from the main service (shared meter, allocated through sub-panel). Optional separate meter for rental ADU coordination — adds $1,500-$3,500 plus utility coordination fees. Plumbing typically shares main service. HVAC dedicated to ADU (mini-split on its own circuit).

Do basement conversions trigger asbestos screening in Boston?

Pre-1978 home garage or basement conversions to ADU require EPA RRP lead-safe procedures. Pre-1981 homes may require asbestos screening on existing pipe-wrap insulation, sheet vinyl flooring, or boiler-room tile. Pro Build crews are EPA RRP certified and arrange asbestos sampling through licensed labs when project age triggers it.

Which payment methods work for ADU contracts?

Pro Build take cash, certified check, business check, and major credit cards. Per Massachusetts HIC law (MGL c. 142A), maximum deposit is 1/3 of contract price, with milestone payments at design completion, foundation/framing complete, MEP rough-in complete, and substantial completion. Third-party financing available from $50,000 to $500,000+.

Do you work with structural engineers I select?

Yes. Pro Build regularly executes designs from outside architects and structural engineers. Single point of execution accountability while preserving the design relationship.

What warranties cover a Boston ADU build?

Six layers of coverage on every Boston ADU: window/door manufacturer warranties (10-20 years), siding warranty (30-50 years), HVAC mini-split warranty (12 years compressor), Pro Build 5-year workmanship guarantee on labor, 780 CMR + HOMES Act compliance verification at certificate of occupancy, and Massachusetts HIC arbitration protection.

Ready to Build an ADU at Your Boston Property?

Free on-site design consultation + 3D layout + line-item written estimate within 5 days for your Boston home. Massachusetts Licensed bathroom pro. No pressure.