Cost to Remodel an ADU Guide
What Does It Cost to Build or Convert an ADU?
Accessory Dwelling Units (ADUs) are one of the smartest ways to add flexible living space, support aging parents, or generate rental income. In Central Massachusetts and Northeastern Connecticut, a complete, code-compliant ADU typically ranges from $150,000 to $420,000+ depending on type (basement, garage/partial remodel, attached addition, or detached), site conditions, and finish level. This guide shows real-world, all-in budgets so you can plan with confidence before investing in design.
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Compare ADU Investment Levels Budget summary for 800 sq ft.
All work done to meet all Massachusetts and Connecticut Building Codes.
*Each budget range does not necessarily include every item listed below it. It is meant to show options that can increase prices in that budget range or that may move you to a higher budget range. Price ranges do not include appliances.
$150K–$220K | Basement ADU
$187–$275 per sq. ft.- Lower-level conversion within existing foundation
- Code egress (window or exterior door)
- Moisture control & insulation upgrades
- Kitchenette, full bath, laundry options
- Mini-split HVAC / ventilation
$160K–$240K | Garage / Partial Remodel
$200–$300 per sq. ft.- Raise/level/insulate slab; build out interior
- Plumbing runs for kitchen & bath
- Upgraded insulation for fire/energy code
- Mini-split HVAC; electrical sub-panel
- Complete finishes (flooring, trim, paint)
$220K–$300K+ | Attached ADU (Addition)
$275–$375+ per sq. ft.- New foundation or slab; roof & siding tie-ins
- Separate entrance & fire-rated assemblies
- Full kitchen, bath, bedroom, living area
- Independent mechanicals as needed
- Code Compliance: Separate entrance and fire-rated walls
- If septic capacity is low: add $10K–$25K+
$260K–$420K+ | Detached ADU (Small Home)
$325–$525+ per sq. ft.- Standalone structure with full utilities
- Utility trenching & separate tie-ins
- Kitchen, bath, HVAC, insulation, finishes
- Zoning: setbacks / parking coverage compliance
- If new septic is required: add $10K–$30K+
Permit, Code, and Soft Costs
Typical Range: $8,000 – $25,000+
- Architectural and structural plans
- Building, electrical, plumbing, and HVAC permits
- Energy code / blower-door testing
- Fire separation and life-safety compliance
Utility & Mechanical Costs
Typical Range: $5,000 – $25,000 (distance and trenching are the big variables)
Includes: HVAC or mini-split, plumbing runs, electrical sub-panel or meter, dedicated hot-water system.
Planning Your ADU in Worcester County & Nearby CT
Every property is unique. Zoning, setbacks, septic capacity, tree clearing, and trenching may change feasibility and budget. CORE guides you through feasibility, zoning review, budgeting/allowances, design, and permitting — then builds your ADU with our Proven Process and 5-Year Transferable Warranty.
Curious which ADU type fits your property and budget?
Schedule a quick design consultation. We’ll review goals, layout options, and a realistic budget range for your home.
Considering other updates to your home? Use the links below to explore what it costs to remodel other areas of your home.
Our Warranty to You CORE Remodeling Services, Inc. warrants all labor performed and material installed at the job site, for a period of (5) FIVE year from date of installation. All workmanship shall conform to the guidelines found in the publication “Residential Construction Performance Guidelines for Professional Builders and Remodelers, Fifth Edition, 2015 , National Association of Home Builders of the United States. If an item is not covered in that publications, standard industry practice shall govern. This may include the dispute resolution process as specified in the contract documents or by applicable laws. This warranty does not include normal wear and tear, and/or product abuse.
Under this warranty, CORE Remodeling Services will be provided the opportunity to have one of its representatives assess any damage identified to be caused by CORE Remodeling Services, Inc employees and/or material installed by CORE Remodeling Services. If said damage is judged to be the fault of CORE Remodeling Services workmanship and/or material, CORE Remodeling Services will at no cost to the Owner repair or replace defective material.
