SMALLER LOTS

If your unit is too large for your household now, you can split it, converting the extra space into an ADU. A two- or three-bedroom unit can become two smaller units: a one-bedroom unit and a studio ADU for instance. The ADU could be used to host a family member with more privacy, be rented out for income, or be home to a live-in caregiver. This scheme is designed to work well with the typical two- and three-unit houses found throughout Boston, which usually have one two- or three- bedroom unit on each floor.

If the building features an unfinished basement and the new ADU is on the first floor, plumbing and electrical updates can be done in the basement without disturbing units above the ADU.

More ways to split your unit

Different versions of this ADU can be adapted to fit your specific building and lot. If you have ample space in the backyard, you could accommodate a larger ADU by enclosing the back porch, therefore increasing the available area for the new unit. Alternatively, if you have a larger side yard, you might consider adding a side extension on the first f loor, which would provide additional room for a one- or two-bedroom ADU.

The example used for the book illustrates an ADU designed for a typical two- or three-unit house. The concept of splitting your unit can also be applied to other types of buildings. For wider building types, for example, splitting your unit to create an ADU could yield a more spacious design for your ADU. In these cases you could also choose to create a different entrance on the front for the new unit. There are many examples of side-by-side duplexes in Boston that can serve as an inspiration.

The Split Your Unit design shown on the previous spreads is designed to work well with classic stacked duplex and triplex buildings like this example found in Boston's Allston neighborhood.

An existing semi-detached home could also split their living space between a unit on the first floor and a unit on the second floor. In most cases, this would require the addition of a second means of egress for the second floor unit, similar to this historic example found in Somerville.

If you have an unfinished attic, you can convert it into livable space, which allows you to create an internal ADU. In this example, the attic space is added to the main unit, and the first floor is converted into the ADU.

Dormers are designed to preserve the original building’s peak roofline, while also creating new living space and windows on the top floor. All units, including the ADU, share a common entrance and have access to shared utilities in the basement. Typically, a second stair will be needed to provide a second means of egress from the upper unit.

The design shown on the previous spreads illustrates the ADU on the first floor and the main unit above. Alternatively, you could convert your attic into an ADU while keeping the original first and second floors as the main unit. To implement this, your architect would need to design in two means of egress from the attic that are separate from the stair within the first and second floor main unit.

There are several ways to alter your home’s roof or add dormers that can increase habitable space and access to natural light. Additionally, depending on local zoning regulations and the maximum allowable height, you might be able to convert your attic from a half-story (pitched roof) into a full story with a f lat roof. This would significantly increase the space available for an ADU on the top floor.

This house in Allston has existing, gabled dormers.

If your building is shorter than those around it, you may be able to add another floor or floors. This kind of addition may be best suited to projects where there are plans to renovate the entire house as you will need to run structural, plumbing and electrical systems down through the rest of the building from the new floor.

Adding a floor will typically mean that your architect will need to follow the commercial building code, which is required for buildings with three or more units. The change from residential to commercial code will trigger bringing all egress routes (stairs) up to the current commercial code and adding sprinklers throughout the building.

The design shown on the previous spread illustrates how adding a third floor to a two-story building can create additional space. A similar approach can be applied to other buildings; for instance, you could add a second floor to a onestory building or an extra floor to a 2.5-story building, depending on the configuration of the units and means of egress. The example in this book shows how adding an additional floor with a flat roof can be done. However, you can also add a floor with a pitched roof and potentially include dormers. In some cases, a pitched roof might be a better option to stay within zoning rules for maximum building height or to blend in with the architectural style of your neighborhood.

If you have a one-unit building and you are adding a floor to include an ADU, you would end up with a total of two units and would be below the three-unit threshold which would require your architect to follow the commercial building code.

This house in Jamaica Plain is one floor shorter than the surrounding context. It could be a good candidate for adding another floor.

One-story homes similar to this one in Allston are rare in Boston, but are ideal candidates for adding a floor or floors.

Many homes in Boston are located on sloped sites or built on elevated foundations. That means their existing basements have tall ceilings and the ability to add and expand windows. These basements have the potential to be converted into an ADU that can host a family member with more privacy, be rented out for income, or accommodate live-in care.

If you are considering converting your basement into an ADU, check if you have room for at least seven feet and six inches of clear space between the floor and ceiling. Building code will also require egress windows from all bedrooms located in a basement apartment. This type of ADU is not possible in any area vulnerable to flooding, such as within the Coastal Flood Resiliency Overlay.

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This approach to adding an ADU can be customized to fit your building and lot. In the example shown on the previous pages, the lot slopes downward from the front to the rear. Therefore, the living area is placed on the side of the basement that gets more natural light. If your lot slopes in the opposite direction, you may want to design the ADU with the living area on the side that faces the front of the lot to maximize natural light. This way, you can ensure that the most light-filled space is positioned where it will be most beneficial.

If you need to use space in the basement for storage or utilities, you can use a portion of the basement for the ADU and keep a portion for other uses. The key concepts will remain the same: you will need to ensure building code-required floor-to-ceiling height, provide code-required egress, update the exterior walls and site drainage to keep moisture out of the basement and upgrade the exterior walls and f loor to meet existing energy codes.

At this house in Mission Hill, access to the basement has been incorporated at the front of the house.

This home, located on a sloping site in Somerville, has added large windows to the above grade portion of its basement facade.

Even very small parcels can fit an ADU. This design shows the addition of a studio apartment ADU on the first floor. By keeping the access to the ADU close to the front door, the existing stair can be used just for the owner’s unit.

This ADU shows an addition in the backyard that lets the homeowner move the main unit’s kitchen and living room into the addition in order to accommodate their new ADU. The new courtyard created between the addition and the ADU can be shared or be reserved for use by either just the ADU or just the main unit.

For rowhouses and other homes with very small side yards, extending towards the backyard is often the best way to add space for an ADU. You can arrange this space in a few different ways. For example, one option is to place the ADU towards the front of the building, which keeps the ADU close to the entrance and allows the existing stair to be incorporated into the main unit. Adding a new courtyard can give the ADU some outdoor space and extra natural light. Alternatively, you could make the existing stair into a common space or throughway. This would allow your architect to position the ADU towards the back of the building, on an upper floor, or in the basement.

If your rowhouse is on a corner lot, you may have more design flexibility. You could place the ADU entrance on the side of the lot, separate from the main unit’s entrance. If you have a corner lot, talk to your architect about different design options and where to best place the entrances to the new unit.

These rowhouses in Mission Hill feature deep backyards and walk out basements - site conditions that could make adding an ADU possible.

Wooden rowhouses and semidetached houses like this Jamaica Plain example may not have existing fire walls. Review with your architect what the design implications may be if that is the case.