How long does an ADU permit take in Boston?

Typically about 18 weeks from complete application to approval. Massachusetts requires both owner-occupancy and parking replacement for most ADUs as of 2025–2026. Boston permit timelines are longest among launch cities — discretionary review possible. Title V septic upgrades common cost driver outside the city core.

How much does an ADU permit cost in Boston?

All-in permit-related costs typically run $8,000–$18,000 (Andalusia Drafting 2026; Dynamic Quality Builders 2026). This includes building permit fees, plan check, and professional soft costs. Impact fees may apply for ADUs over 750 sq ft.

Do I need to replace parking when building an ADU in Boston?

Yes — Boston requires replacement parking for most ADU projects. Verify the current rule with the city planning department.

ADU permits · Boston, Massachusetts

Boston ADU permit fees, timeline, and rules

Sourced permit fee ranges, typical approval timelines, and the ADU-specific rules that apply in Boston. All numbers cross-referenced against published builder data and the city's planning department.

Last validated by OwnAdu: May 15, 2026 (manually verified)

Typical permit fees (all-in)

$8,000 – $18,000

Typical approval timeline

~18 weeks

Maximum ADU size

900 sq ft

Parking replacement required

Yes (check city map)

Owner-occupancy required

Yes

Setback requirements

Varies by zoning subdistrict

Source: https://www.boston.gov/departments/inspectional-services/accessory-dwelling-units

Buying a home with an ADU in Massachusetts?

State law, typical premium, what to verify on a permitted ADU, and what to do if you find an unpermitted one.

Massachusettsbuyer's guide →

Get a real quote for your Boston project

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Get sourced ADU permit updates monthly

Permit fees and timelines change. We re-validate against the source URL above each quarter.