Over the years I waffled once or twice about adding a third unit, in the basement. I hired a structural engineer to draw up plans for the replacement center beam in the basement’s ceiling. To “future proof” the plans, I also asked him to draw a plan and create specifications for a floor excavation in order to achieve a code-compliant ceiling height in the potential basement unit.
The waffling was based on the cost of the necessary digging down. Excavation will add about $25,000 to the cost of the project. (That’s based on one bid, the only bid that called it out specifically. I asked all three bidders to separate the costs of the basement unit so that I would know exactly how much the basement unit – excavation, walls, finishes, etc. – would cost as an “add on”.)
I think my architect also helped convince me to design and get bids for a third unit because of the flexibility it could provide for me and my tenants (which may include one or more family members), as well as the possibility of additional rental income.
I’ve settled on adding the third unit, and I’m excited to do it, in part because of my well-known enthusiasm for accessory dwelling units. (A note about Chicago’s ADU ordinance: this property is outside the ADU pilot area but it has unused zoning capacity per both the FAR and minimum lot area per unit standards so the additional unit is allowed without the ADU ordinance.)
Excavation
The Chicago Building Code requires that the ceiling height be a minimum of 7′-6″ in most of any dwelling unit (that’s measured from the finished floor to the finished ceiling). A limited amount of area is allowed to be lower to get around existing or newly installed pipes and ducts in the ceiling.
To achieve that, crews will demolish and remove the existing floor slab (which I’ll be a little sad about because the previous owner did a nice job pouring it) and excavating earth 10″ down. They’ll also underpin the foundation walls in an alternating pattern of rectangles around the perimeter. I hope I can explain underpinning well (otherwise watch Darren Voros’s video): This means they’ll dig under half of the foundation dispersed around the perimeter (the A sections), leaving unexcavated space between each to hold up the other half of the foundation (the B sections); then they’ll switch and repeat the process for the B sections.
Water
After every rain storm I go and check the house. The basement has been dry every time. I’m surprised, and thankful, that it seems that water doesn’t seep into the basement.
One time I noticed that there was water pooling on the basement floor – when a rain storm had not occurred. It turned out that when I closed the water service line I didn’t close it well enough and water was very slowly leaking out. Tightening the valve handle was a simple resolution.
During the excavation there will be a perimeter drain tile system installed under the floor slab to steer water toward an ejector pit. An ejector pit is different than a sump pump in that it has a mechanism to grind the house’s sewage before pumping it to the city’s sewer main. I believe an ejector pit is a requirement when the lowest dwelling unit’s plumbing (i.e. the toilet) will be below the city sewer main.
Bob Vila’s website describes interior drains like this:
Similar to exterior drain tile, an interior French drain features a perforated pipe that carries water to a collection pit where it can be pumped to the surface. This type of drain is located along the interior perimeter of the basement and lies below floor level.
All You Need to Know About Basement Drains, Glenda Taylor and Bob Vila
Apartment
The new dwelling unit is drawn as a 1-bedroom apartment with a small bedroom that has two short windows and a closet. It has a large, combined living room, dining room, and kitchen.
The entryway is partially separated from the big room using a hall closet, creating a sort of mud hall. The bathroom is of a typical size for small apartments in Chicago and there’s a considerably sized utility room for laundry and storage. Overall, the apartment has an area of about 720 s.f.
I believe in-unit storage to be an important amenity. My current apartment is a studio and I selected it in part because it had good layout with a lot of scattered storage (there’s a walk-in closet, a large nook between the living area and the bathroom, a small closet, and a linen closet in the bathroom).
When I was looking for a new place to live in 2022 I didn’t want to have to move any of my stuff into a storage locker in the building or a storage unit in another building.
Other space in the basement
The rest of the basement, about 100 s.f., will have two hybrid heat pump water heaters, the ejector pit, electrical panels, and a little bit of room for storage. There’s also a rear entryway and the basement apartment will have a rear exit through here.