Cost to ADU / In-Law Suite Construction in Boston, MA

$79,200–$330,000 in Boston

32% above the national average · 1.32x cost multiplier

In Boston, a adu / in-law suite construction runs $79,200 to $330,000 on average — about 32% above what homeowners pay nationwide. The Northeast market's labor costs account for most of the gap, with material pricing and local permitting fees adding to the spread. Use the cost table below to compare budget, mid-range, and premium options side by side.

Cost Breakdown in Boston

Line Item	Low	Mid	High
Cost per sq ft	$198	$330	$528
Materials	$36,960	$85,800	$158,400
Labor	$42,240	$92,400	$145,200
Permits	$5,280	$10,560	$19,800
Design	$6,600	$13,200	$26,400
Total	$91,080+32% vs. national avg	$201,960+32% vs. national avg	$349,800+32% vs. national avg

Budget

Garage conversion ADU (400 sq ft) - kitchen, bathroom, bedroom, electrical upgrade, no exterior changes. Most affordable ADU type.

Mid-Range

Detached backyard ADU (600 sq ft) - new construction on concrete foundation, full kitchen, bathroom, bedroom, separate utilities, standard finishes

Premium

Custom detached ADU (800+ sq ft) with high-end finishes, vaulted ceilings, full kitchen, 1.5 bathrooms, separate utilities and address, premium landscaping

Material Options

Material prices below are national averages; availability in Boston may shift costs slightly.

Material	Cost/Unit	Pros	Cons	Best For
Garage Conversion ADU	$80 - $200 per sq ft	Lowest cost, structure exists, fastest permitting in many states, minimal site work	Requires losing garage parking, typically smallest footprint, attached to main house	Homeowners with detached garages who want the lowest-cost path to rental income or multi-generational livingLowest cost, structure exists, fastest permitting in many states, minimal site work
Basement Conversion ADU	$100 - $225 per sq ft	Uses existing space, natural privacy from main house, lower cost than new construction	Requires egress windows, can feel cave-like without proper light, limited if basement has low ceilings	Homes with full-height unfinished basements in markets where ADUs are in demandUses existing space, natural privacy from main house, lower cost than new construction
Above-Garage Addition ADU	$175 - $325 per sq ft	Separate from main house living area, good privacy, no lot coverage increase	Requires structural assessment of existing garage, staircase required, more complex than conversion	Properties where lot coverage is maxed out or where separation from the main house is a prioritySeparate from main house living area, good privacy, no lot coverage increase
Prefab / Modular ADU	$150 - $300 per sq ft (including site work)	Faster construction, predictable pricing, some high-quality options, off-site quality control	Still requires permits and full site work, delivery and placement costs add up, limited customization	Markets where prefab ADU companies operate and homeowners who want faster construction timelinesFaster construction, predictable pricing, some high-quality options, off-site quality control
Custom Stick-Built Detached ADU	$200 - $400+ per sq ft	Fully customizable, can match main house architecture perfectly, highest long-term value	Most expensive option, longest timeline, most complex permitting	Homeowners who want a long-term investment that looks like a natural part of the propertyFully customizable, can match main house architecture perfectly, highest long-term value

Other Projects in Boston

Full Bathroom Remodel

Full Kitchen Remodel

Window Replacement

Back to national average for ADU / In-Law Suite Construction

Boston Estimate

Typical Range

$79,200–$330,000

vs. National Average

32% above national

Cost Multiplier

1.32x

Cost Per sq ft

$198–$528

Estimates apply Boston's 1.32x regional cost multiplier to national averages. Actual bids vary by contractor, project scope, and timing.

See national average costs →