Planning A Basement - Rule Of Thumb

​Basements sound like a really good value -
​residential, yes; commercial, no.

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In the Midwest and probably just about everywhere else with workable soil conditions, a house basement adds minimal cost for the additional raw square footage. The additional cost is pretty much limited to a stair, slab-on-grade, 4’ higher foundation walls and damp proofing with a footing drain. Without finishes this is pretty minimal. It's a good deal. The cost per SF is about 10-15% of the SF cost on the rest of the house.

This knowledge doesn't transfer to commercial buildings at all. But that doesn't keep nearly every client from wanting a basement in their office building, school, church...

The commercial reality is that an unfinished basement will cost 75% of what you are spending on the rest of the building. First lets look at the extras for a house in terms of a commercial building.  

Stair: As a means of egress the stair will need a fire-rated enclosure. The stair itself will be larger because of the commercial tread/riser ratio, minimum width. It may not even be wood. And you may need two of them.

Slab-on-grade: The ground floor of a commercial building was going to be a slab-on-grade. Now you have moved it to the basement, and you have added a structural floor in its place. The structural floor is about five times as expensive.

4’ higher foundation walls: Story heights are greater in a commercial building so you have not only added about eight feet instead of four but the wall thickness and reinforcing have increased as well.

Damp proofing with a footing drain: Because of the taller basement wall, waterproofing is often called for. It is just too critical to hang your hat on damp proofing.

But of course that is not all.

• Fire-Rated-Ceiling: Either a drywall ceiling or a fire-rated ceiling is almost always required by code for either a basement or a storage area.
• Sprinklers: A basement is required to be sprinklered in the International Building Code even when the rest of the building is not.
• Emergency and exit lighting: this isn't major, but it adds cost.
• HVAC: at the very least you will need space heaters and ventilation. Without some system that can be used in the future, it will be hard to air condition the basement when it is finished off.

And where do you draw the line on partitions (for security of stored goods), future toilet rooms, future doors and windows, and electrical infrastructure.

I haven't ever had a client who thought that the real cost of a basement was worth it. Just quote the cost up front and save yourself the design time.