Cost of adding a basement to a new addition
A designer in our network is working with a client who is considering adding a 400-square-foot basement below a new 400-square-foot addition. They are debating whether it is worth the added cost in order to gain added usable area.
Basements are standard for many parts of the country as they provide additional usable space. However, in some areas, like California, they are much rarer. While they’re traditionally used for storage and to house appliances such as washers and dryers, furnaces and water heaters, they can also be finished and used as home offices, additional living spaces, or even ADUs. Of course, with this added space comes additional cost.
To excavate a 20′ x 20′ space will cost over $20,000. This includes the equipment rental, labor, excavation work and removal and disposal of the 120 yards of soil.
Exporting soil has a fee of about $750 a load. A load in a dump truck is 7-8 yards.
20′ x 20′ x 8′ deep is about 120 yards of soil. That is 17 loads and thus about $12,750 in fees to the transfer station/dump.
5 days of labor plus machinery is $1,500/day = $7,500 for labor and machinery rental.
Shoring is additional and can vary depending on the site and soil conditions.
So now you have spend about $20,000 and have a hole in the ground. Next we need to add rebar and pour concrete for the footings, basement floor slab, and foundation walls. This work will be about $19,000 based on recent quotes we have received.
At this point we are around $40,000 in total cost for the basement space of 400 square feet. That is about $100 per square foot just for the site work, excavation and concrete work for the basement. This is before you add window wells, stair access, and utilities, or other aspects to finishing the interior of the basement to make it usable space.
This is still a very good value and a wise investment if your property can accommodate a basement. Around the San Francisco Bay Area, real estate values square footage at about $900 or more per square foot, and final construction costs for a finished basement can end up falling between $300–$500 or more per square foot. This shows it is a good long-term investment and makes adding a basement a wise decision when adding an addition or ADU to your property.
Once you decide to move forward with adding a basement to your project, it is wise to consider options and amenities that can really make this a great space to live in and use despite it being a basement:
Add radiant floor heat.
Get a bedroom or two into a 400-square-foot footprint and rent it out.
Have a basement bar.
Add a playroom for the kids.
Put in a home office.
Make it a media room.
Make a sound insulated music room.
Add ample window wells for fresh air and natural light
Add exterior stairs so you can access the basement from the backyard or without going through the main house (a separate entrance if it is a rental unit or guest suite).
If you’re adding on to your home, a basement could be just what you need to make your improved space even better.
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