ADU Regulations:
Waltham
Regulations Snapshot
Rear and Side setbacks: 5 feet
Building Separation: 10 feet
Height: cannot be taller than principal house
Separate utilities required: Yes
Stormwater management: Yes
Specialized Stretch Energy Code: No
Impact Fees: $750 per bedroom, $200 for water/sewer
Building Constraints
Number of ADU’s allowed on a single family lot
Only one ADU is allowed per single-family lot, whether interior, detached, or attached
Maximum square footage of an ADU (detached or attached)
The minimum floor area of an ADU shall not be less than 450-square feet. The maximum floor area of an ADU shall not exceed 900-square feet, or 50% of the total gross floor area of the principal dwelling unit, whichever is smaller.
Maximum Height
existing principle house
Maximum lot coverage
goes by underlying zone 25 or 30%
Exterior appearance requirements
Yes must be similar style and appearance to main house
Setbacks
Rear setback
5 feet (setbacks fall under accessory structure dimensions)
Side setback
5 feet
Front setback
even or behind main house
Distance between buildings
10 feet from primary dwelling
Parking
Parking requirement for an ADU
Each ADU must include one dedicated off-street parking space unless the property is located within half a mile of a public transit station.
Where can the parking be located : Parking requirement for primary dwelling
Two off-street parking spaces
Additional Requirements
Separate utilities required
Need to run all the way to the main in the street. The contractor performing the work needs to come from an approved bonded and licensed list.
Stormwater management required
Need a leaching pit to capture water from new impervious surfaces. Don’t need engineering for a single family owner occupied property, the town provides premade plans.
Town impact fees
$750/bedroom through the engineering department. $200 for water/sewer hookup
Soils report required
inspect hole then maybe geotech
Civil plans required
Yes
Short term rental allowed
31 day minimum
Possible overlay districts
If your property is within a historic overlay, it may trigger design reviews or special permit requirements for an ADU. If your property lies within 100 ft of wetlands you’ll need to file with the Conservation Commission before building. Properties in watershed and floodplain zones may face restrictions on impervious areas.
ADUs on multifamily lots
The City Council may issue a special permit to allow more than one Accessory Dwelling Unit (ADU) on a single property, provided the following conditions are met:
- Compliance with ADU Standards: Any additional ADU must meet all applicable requirements outlined in Section 3.23 of the zoning bylaws.
- Design Compatibility: The architectural style and exterior features of the ADU must align with the primary residence and blend with the character of the surrounding neighborhood.
- Fire Safety Requirements: If the total number of dwelling units on the property (including the main home) reaches four or more within one structure, a sprinkler system must be installed in accordance with fire safety codes.
- Parking Requirements: Each additional ADU must include one dedicated off-street parking space on the same parcel as the main home, unless the property is located within half a mile of a public transit station.