English basement accessory apartment in DC by Daniel Warwick.

You may be familiar with the typical “English basement,” an apartment at the bottom of a rowhouse that's particularly popular with interns and small families in DC. They’ve been around for decades, but now they’re gaining even more popularity as the result of housing shortage and other market realities, as well as the desire to live in close-knit communities.

English basements are one way to add more affordable housing in neighborhood, but there are other types of add-on apartments homeowners can build and rent out as well. Converted carriage houses, “granny pods,” and “nanny flats” are other types of accessory apartment, also known as Additional Dwelling Units, Accessory Dwelling Units, and ADUs.

So can you create an accessory apartment on your property? Are you curious about whether your finished basement constitutes an additional dwelling unit or not?

What does it take to set up an accessory apartment?

As an architect, I visit properties to evaluate the feasibility of implemeting an accessory apartment within the existing building. I've compiled a list of some basic things to look for.

Aside from design or economic considerations, the building code outlines very strict guidelines on what needs to happen for an accessory apartment to become a legal dwelling unit. The Department of Consumer and Regulatory Affairs (DCRA) follows the International Building Code and the International Residential Code from which these rules were extracted. These rules are the consequence of long historical debates about what constitutes safe and fair housing, and most jurisdictions in the Washington region follow them.

One of my designs for a basement accessory apartment. Image by the author.

When you walk through your building wondering how to implement an accessory apartment, look at the following items and details. If these items aren't up to standard, alterations are often feasible. The cost will vary based on the existing conditions and the desired features of the future accessory apartment.

1. Ceiling height

Before starting to build or retrofit, check the existing ceiling clearance. The building code is very strict on the 7’-0” minimum ceiling clearance for habitable spaces such as bedrooms and living rooms. Non-habitable spaces such as bathrooms and kitchens can be 6’-8”. Ceiling clearance below ducts and beams can be as low as 6’-4”.

How should you figure out ceiling clearance? Measure from the top of the finished floor (for example, the tile surface of the kitchen) to the bottom of the drywall on the ceiling. If you have the proper clearance, good! if you don't, your architect will help you figure out an effective and economical way to reach it.

Whatever your existing or desired ceiling height, make sure you document it property to show the permitting agency how you plan to comply with the code. Once you build or retrofit your basement, the city inspector must verify that the ceiling clearance complies.

2. Exits and access

Egress (or places to exit) are fundamental because residents must be able to leave the building in the case of emergency, and emergency personnel being able to get inside. Each unit needs a minimum of two points of egress.

The main entrance door is the first point of egress. It needs to open to the public way, and/or be easily accessible to the public way. The secondary exit point must be a window that complies with size requirements and is able to open. Aside from the width, height, and sill height, the window must not have fixed bars, among other rules.

3. Fire separation

Additional dwelling units must maintain a one-hour fire separation between units. This is easily accomplished with a continuous drywall ceiling that complies with the building code. Depending on the existing conditions of the structure, there are various options on how to implement a fire separation.

Also, all items penetrating the separation, such as the recessed light fixtures or pipes navigating both units, must be fire-stopped. Smoke detectors must be provided for both units. Depending on the building type, location, and scope of work, fire sprinklers may be required as well.

4. Heating and cooling

The main residence and the accessory apartment must have separate control of their comfort levels. Heating and cooling equipment must be accessible to the residents it serves and allow for independent thermostat setting. Depending on the sophistication and location of the mechanical system, the equipment could be shared between units, so long as each resident can access it without intruding into the other dwelling.

5. Electrical system

Just like the mechanical system must be controlled by each resident, same goes for the electrical service. Each unit will need an electrical panel, and all circuits of each unit must be fed from its own panels. Electrical services must not be mixed.

Separate electrical meters are also recommended to make it easier to measure how much each unit consumes. For houses with photovoltaic panels, the solar electricity will only feed one of the two dwellings.

6. Plumbing system

Just as mechanical and electical systems must be controllable and serviceable by each resident, same goes for the plumbing. Each dwelling must have its own means to heat water and distribute it. Each unit must have a control valve as well.

So what can be shared? The waste lines. Since basement accessory apartments are separated from the main dwelling, the main waste pipes can be joined.

7. Area distribution

An additional dwelling unit is not a tiny house. Period. An additional dwelling unit must comply with all areas and clearances to constitute a safe space to live. Stairs, railing, corridors, and habitable spaces must follow the minimum and maximum rules set by the International Building Code.

It's worth noting that most tiny houses featured on TV do not comply with area distribution (among many other) building code requirements.

8. Kitchen and bathrooms

In order to constitute an accessory apartment, it must contain the space and equipment to store, prepare, and serve foods in a sanitary manner. The kitchen must have a sink, cooking, and refrigeration appliances. The code does not outline the size or capacity of the kitchen equipment. A two-burner stove may be enough, so long as it’s hardwired to the electrical system as required by the code.

The bathroom must have a bathroom, sink, and shower. Again, there are minimum clearances between the plumbing fixtures but the sizes of each piece are not set by code, just by the market. A washer and dryer are not mandated, but it’s a good idea to add them.

9. Green code

DC's regulating agency, DCRA, follows the green code. This means that all construction above a minimal scope—and an accessory apartment exceeds the description of minimal scope—must comply with the regulations set by the International Energy Conservation Code. This code sets rules for insulation, fenestration (windows and doors), fresh air intake, and heating and cooling, among many other requirements.

10. Bedrooms

Of all the code questions that come up, the one that trips most people is “what constitutes a bedroom?” Aside from market and desirable features, you must have the following items if you want to call a certain space a “bedroom.” (Dens, offices, etc. are not bedrooms.)

  1. 70 square feet, with 7’-0” minimum length or width
  2. 7’-0” ceiling height
  3. An egress window
  4. A hardwired smoke detector
  5. Hardwired or fixed heaters, like a radiator or a baseboard

Are we missing anything?

How about a closet? Actually, there are no building code provisions for storage (except for storing food in a refrigerator in the kitchen). You can put a wardrobe in a 70-square-foot bedroom, and it would suffice. You could have a walk-in closet as an added feature for market value, but so long as you comply with points 1-5 in item #10 above, your space is a bedroom per the International Building Code.

The items outline the regulations that must be incorporated if you plan to build an accessory apartment in your property. If you're concerned about meeting them, know that there a wide variety of technical and creative solutions to implement each requirement. The regulations don’t negatively affect the efficiency of the space—and there are years of beautiful design for small urban spaces to prove it.

Readers: What questions do you have about building accessory apartments?

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Argentina-born Ileana Schinder is a licensed architect. She holds a B.Arch in Architecture from Universidad Nacional de Córdoba (Argentina) and an MA in Public Communication from American University (Washington, DC). Ileana has nurtured her passion for architecture since first hearing the click of LEGO bricks at the age of four. She lives in Washington, DC with her family and Cecilia, the dog.