Accessory Dwelling Unit Webinar - Jun 12th, 2024

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"What's an ADU and how do I build one?"

Webinar held via zoom. Meeting information was available via This event was a joint effort by Arlington's Department of Planning and Community Development and Equitable Arlington.

(Alex Bagnall, Envision Arlington) Mr. Bagnall says that Arlington's Town Meeting voted to allows Accessory Dwelling Units (ADUs, aka "granny flats") in 2021. An ADU is self-contained housing on the same lot as a principal dwelling.

(Phil Tedesco, Goulson Storrs) Mr. Tedesco says the 2021 bylaw was Arlington's third attempt at passing an ADU bylaw -- it was a three-year effort. Mr. Tedesco was part of the group that worked on the 2021 bylaw, and they spent a lot of effort looking at what other communities had done, in order to see what worked and what didn't. Some municipalities impose a lot of restrictions, which makes building an ADU impractical. The state's Housing Choice Legislation provided a framework for what an ADU should be.

Mr. Tedesco says that Arlington's ADU bylaw generally follows existing zoning. They're allowed as an accessory use to single- and two-family homes, and duplexes. The size is limited to 900 square feet, they don't require additional parking, they can be attached or detached, and there's a carve-out for non-profits that want to use ADUs to provide affordable housing. There are twelve ADUs built or under construction in Arlington, which works out to around four per year. The Massachusetts Legislature is also considering statewide ADU legislation.

(Erica Schwarz, Housing Corporation of Arlington) Ms. Schwarz is the Executive Director of the Housing Corporation of Arlington and they're building an ADU on their property at 40 Dorothy Road. They worked with Reframe Systems; Reframe is a startup and were willing to give HCA a special price point for an affordable ADU. Their ADU is 100% electric, passive house certified, and was built with pre-fabricated modular units. Reframe's factory is in Andover, Massachusetts. The ADU replaced a deteriorating garage on the property.

Ms. Schwarz says she learned that Arlington's ADU bylaw doesn't provide a lot of flexibility; HCA had to meet all of the requirements in the bylaw and there was no room for negotiation.

The ADU is family-sized -- 900 square feet with two bedrooms. The initial rent will be $1,670/month for a household making 60% of the area median income.

HCA will examine other sites to see if another would be suitable for an ADU. She would welcome contributions of land for affordable housing, and is open to partnering with individuals.

(Pat Hanlon, ZBA Vice-chair) Mr. Hanlon says the ADU bylaw doesn't excuse compliance with the rest of the Zoning Bylaw. The inflexibility that Ms. Schwarz described is part of the Zoning Bylaw itself, and not specific to ADUs.

Detached ADUs can be built within six feet of a property line, if the ZBA finds that it's not substantially more detrimental than a private garage or other allowable use. This lot line exception only applies to detached ADUs, and the fire rules for garages are not necessarily applicable.

Mr. Hanlon explains the difference between a special permit and a variances. Special permit uses are allowed, but they get a closer look. Variances are actual exceptions to the bylaw, due to specific types of hardships. They're very rarely granted because the requirements are strict.

Mr. Hanlon says that ADUs have to meet all applicable building and fire codes, which are administered by the Inspectional Service Department (not the ZBA). So far, there have been 8 special permit hearings for ADUs; five were approved and three were withdrawn.

(Ed McDonald, McDonald Contracting) Mr. McDonald is a local contractor, and he describes the different forms that an ADU can take. There are three general types: internal, attached, and detached.

Internal ADUs include garage, attic, and basement conversions; they're part of the main dwelling. Internal ADUs typically offer reduced construction costs, easier utility hookups, fewer regulatory hurdles, and faster construction. They may provide less privacy, and there will be disruption during construction.

Attached ADUs include side and dormer additions, which are attached to the main house. There's usually limited impact on the existing footprint and reduced structural construction costs. However, building an attached ADU generally means giving up some exterior space on the property.

Detached ADUs include detached garage conversions and tiny homes. They're more private but tend to have higher construction costs.

Mr. McDonald goes on to compare the general costs of these three options. He says the costs presented are only representative, as there are many factors involved.

Internal ADUs (e.g., converting an unfinished basement) might cost $200k. Attached ADUs might cost around $250k, assuming they're built as an addition on a new foundation. A detached garage conversion might cost $265k.

(Chris Scalzilli, ADU Owner) Mr. Scalzilli build a detached ADU to accommodate an aging family member. It's a converted garage with 748 square feet. Mr. Scalzilli says the cost was in the range of $130--150/square foot.

(note: multiplying that out gives a total cost of $97k to $112k.

He bumped out a one-car garage, and it was around a four-month process.

(Mike Ciampa, Director of Inspectional Services) Mr. Ciampa says that ADUs are an accessory use that's allowed in conjunction with single- or two-family homes, but not three-family or larger. Detached ADUs require natural light and ventilation, two means of egress, smoke and CO2 detectors, and a washing machine hook up. Building an ADU doesn't change the underlying single- or two-family zoning.

Building an ADU in a single-family home requires fire rating between the two units. In a two-family dwelling, an ADU requires sprinklers and alarms in addition to fire separation -- it has to follow the commercial building code.

ADUs do not require dedicated off-street parking, they cannot be used for short-term rentals, and cannot be owned separately from the main building.