Basements in Los Angeles? Yes.

You don’t often see basements in Los Angeles, but they can be an efficient way of adding square footage when there are tight zoning-code limits on the size of a house. Under current L.A. zoning, a basement’s area is completely excluded from a property’s maximum square footage restrictions. A basement can be a reasonable way of adding square feet, if conditions are right.
In recent years there’s been an effort in Los Angeles to limit the size of homes. The most recent zoning changes include two new ordinances: the BMO (Baseline Mansionization Ordinance) for non-Hillside areas, and the BHO (Baseline Hillside Ordinance) for Hillside-zoned houses.

The new ordinances can be very restrictive compared to the previous code limits. We’ll use, as an example, a 7,000 square-foot lot in a Hillside R1 zone. Let’s assume the lot is sloped at 15% (hillside restrictions increase with the steepness of the slope). For this lot (and slope), the zoning code allows a house no larger than 3,150 square feet (not including a 200 square-foot credit for a garage, and other exclusions).

Many Hillside-zoned properties are larger, and have even tougher restrictions: a 15,000 square-foot lot in a Hillside RE15 zone, with the same 15% slope as the earlier example, would be allowed a 4,500 square-foot house (not including the exceptions mentioned above).

This may seem–and is–large. But even a modest remodel or addition to a larger, older home can push the final size beyond the allowable limits. The situation gets even more complicated for smaller lots, and for steeper ones where limits are more stringent still.

For owners looking to expand, there is a solution with few constraints: a basement. Under current L.A. zoning, a basement’s area is not included in a property’s maximum square footage calculations. As long as a basement does not extend more than two feet above existing grade, it can be built anywhere on a lot, up to the limits of property-line setbacks–with no other limits on size.

Say you’re building a new two-story house on the RE15 lot described in the example above (15,000 square foot lot, 15% slope). If the basement is kept entirely within the boundaries of the new house, the size of the house can now increase from 4,500 square feet to about 6,750 square feet (because you are effectively adding one more story, underground). And if the basement extends beyond the house’s footprint, it can be even larger.

You can build a basement under an existing house, or–counterintuitively– completely outside of it. One of my firm’s projects included a large basement under a spacious front yard; it overlapped the house just enough to include a staircase from the main floor down to the basement.

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If you’re building a new addition attached to the exterior of an existing house, one option is to limit the basement just to the area of the addition, which may then include three stories: a two-story addition plus basement. This is an efficient arrangement for a new basement because you avoid major changes to the existing house (and no excavation beneath it). In this case the basement could also have a larger footprint than the above-ground addition, extending out under the yard, all the way up to the property line setback if desired.

Now the caveats: basements can be costly to build. Excavation is expensive. Waterproofing and drainage must be executed with care and precision. If the basement is built beneath an existing house, the existing structure must be altered to accommodate it. Daylighting and exiting are important, code-mandated considerations, and the architect must plan them with care. If a bathroom or kitchen are included, the architect and contractor must plan to get the waste to the sewage line, even when the basement is lower than existing sewers. In that event, a sewage ejector pump is required, to get the waste up to the required level.

None of these items involve heroic measures. The solutions are well understood, widely used, and the equipment and materials easily available (ejector pumps, for example, are available at Home Depot). And although they can be expensive, basements are worth building if the conditions are right.

Here’s a brief rundown of advantages and challenges to basement construction:

Advantages
• Additional square footage with few restrictions.
• Very flexible space: storage, or bedrooms, wine storage, offices, etc.

Challenges
• Cost
• Increased complexity (waterproofing, exit requirements, ventilation, lighting and plumbing).
• Soils conditions can have a significant impact on construction costs.