How Much Does an 800 Sq. Ft. ADU Really Cost in Worcester County, MA?
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Example of a garage-to-ADU conversion — one of several options for Worcester County homeowners exploring accessory dwelling units.
Realistic Breakdown for Homeowners
7-Minute Read
Homeowner Education Guide
Accessory Dwelling Units (ADUs) are growing rapidly throughout Central Massachusetts as families seek in-law suites, rental income, or private space for aging parents.
But just like with new home estimates, most online pricing only reflects the structure — not the full, real-world project.
In reality, a complete, code-compliant 800 sq. ft. ADU in Worcester County typically costs between $150,000 and over $420,000, depending on the type, site conditions, and finishes.
What Is an ADU?
An Accessory Dwelling Unit (ADU) — sometimes called an in-law suite, backyard cottage, or secondary apartment — is a self-contained living space on the same property as a main home, with its own kitchen, bathroom, and entrance.
They’re one of the most flexible ways to create housing for extended family, generate rental income, or increase long-term property value.
Purpose of This Guide
Our goal is to help Central Massachusetts homeowners understand what’s truly involved — financially and logistically — in building an ADU, so you can plan with confidence before investing in design or construction.
Realistic All-In Budget Summary for 800 Sq. Ft. ADUs
These figures represent true, complete project costs — including design, permits, utilities, finishes, and construction.
ADU Type Realistic Total Cost Range Cost per Sq. Ft.
Basement Conversion $150,000 – $220,000 $187 – $275 per sq. ft.
Garage / Partial Remodel $160,000 – $240,000 $200 – $300 per sq. ft.
Attached Addition $220,000 – $300,000+ – $275 – $375+ per sq. ft.
Detached ADU $260,000 – $420,000+$325 – $525+per sq. ft.
1. Basement ADU (Lower-Level Conversion)
Basement conversions are typically the most affordable option since the structure, roof, and foundation already exist.
Typical Range: $150 – $250 per sq. ft.
800 Sq. Ft. Example: $120,000 – $200,000
Key Cost Drivers
Egress Requirements: Adding code-required egress windows or an exterior door
Moisture Control: Drainage, vapor barriers, and waterproofing
Life Safety: Fire separation from the main home
Systems Integration: Ventilation, HVAC, and fresh-air requirements
Insulation: Framing and insulating concrete walls
Realistic All-In Range: $150,000 – $220,000 (including egress, plumbing, HVAC, and finishes)
2. ADU Home Addition (Building Out from the Main Structure)
An attached ADU is essentially a new mini-home connected to your main house — ideal for convenience and accessibility.
Typical Range: $250 – $350 per sq. ft.
800 Sq. Ft. Example: $200,000 – $280,000
Key Cost Drivers
New Foundation: Full foundation or concrete slab
Structural Tie-Ins: Framing, roofing, and siding integration
Independent Systems: Kitchen, bath, and mechanicals
Code Compliance: Separate entrance and fire-rated walls
If septic capacity must be expanded: add $10,000 – $25,000+.
Realistic All-In Range: $220,000 – $300,000+
3. Garage Conversion or Partial Remodel
Often more affordable than a full addition, depending on structure and utilities.
Typical Range: $180 – $275 per sq. ft.
800 Sq. Ft. Example: $144,000 – $220,000
Key Cost Drivers
Slab Work: Raising, leveling, or insulating the floor
Plumbing Runs: Adding bathroom/kitchen lines
HVAC: Installing ducting or mini-split systems
Fire & Energy Code: Upgrading insulation and safety barriers
Realistic All-In Range: $160,000 – $240,000 (including HVAC, bath, kitchen, and finishes)
4. Detached ADU (Separate Small Home)
Detached ADUs offer full independence and privacy — but they cost the most since they’re standalone homes.
Typical Range: $300 – $500+ per sq. ft.
800 Sq. Ft. Example: $240,000 – $400,000+
Key Cost Drivers
New Structure: Foundation, framing, and roofing
Utility Trenching: Separate plumbing/electrical tie-ins
Complete Systems: Kitchen, bath, HVAC, and insulation
Zoning Requirements: Parking or setback compliance
If a new septic system is needed: add $10,000 – $30,000+.
Realistic All-In Range: $260,000 – $420,000+
Permit, Code, and Soft Costs
Soft costs are the behind-the-scenes professional expenses that make your ADU legal and safe.
Typical Range: $8,000 – $25,000+
Includes:
Architectural & structural plans
Building, electrical, plumbing, and HVAC permits
Energy/blower-door testing
Fire separation and life-safety compliance
Utility & Mechanical Costs
These cover the physical tie-ins to make the ADU fully functional.
Typical Range: $5,000 – $25,000** (based on trenching and distance)**
Includes: HVAC or mini-split, plumbing runs, electrical sub-panel or meter, and a dedicated hot-water system.
Why Homeowners Choose ADUs
Rental income potential
Aging-in-place or multigenerational living
Lower cost than buying another property
Adds long-term resale value
Creates privacy and independence
Many homeowners use home-equity loans, construction loans, or rental income projections to help finance their ADU.
Ready to See What’s Possible on Your Property?
At CORE Remodeling Services, Inc., we specialize in complete ADU design-build projects across Worcester County and Central Massachusetts.
We can create a custom ADU Budget Snapshot based on your property type, zoning, and desired finishes — so you know exactly what’s financially realistic before you invest in plans.
