Summary: Building a home addition with a garage underneath is a great way to save money on a basement garage.

*“Carl, I have thought about building my own home addition here in NH.*

*I’m confident that I could build it all myself minus the concrete foundation.*

*I would like a rough guesstimate on the cost to build a 24’ X 20’ home addition with a garage underneath.*

*Thanks for any help, George”*

Hi George,

If you have a sloping lot, building a garage under your home addition is a great way to save on building costs as you are going to have a basement anyway (or at least a large crawl space).

If you don’t have a sloping lot, I would not put the garage underneath the home addition. Besides the cost of a concrete or masonry basement being greater than for a wood framed garage above grade, the logistics of planning for the garage entrance and for proper water drainage on flat land could be more difficult and more expensive as well. Also, be sure to read my **Caution** footnote at the bottom of the page.

Here is an attractive design with a garage or basement under a new addition. Estimating a 24 ft deep and 20 feet wide basement and main floor, and a finished upstairs area of approximately 240 Sq ft I came up with a total living area of 720 sq. ft. and a garage or an unfinished basement of 480 sq. ft.

*Addition house plan courtesy of CADSmith Studio*

Run the construction estimating software found under # 5 on my “Getting Started” page for a 720 sq ft house for your zip code in NH.

Go all the way through the cost estimating process for building the whole house addition utilizing the following information for the Building Calculator (Estimating Software):

The perimeter of the building has 4 corners.

The living area is 720 square feet.

The number of stories is 2.

Basement: 480 sq ft of unfinished basement.

Central ducted heating and cooling.

When you come to the question, “Is there a basement?“ enter 480 in the box for ”unfinished basement”.

When you get to the question “Is there a garage?” leave the boxes empty.

When you have completed the cost estimate you will have not only the cost of materials (and labor if you decide to sub it out) for the addition, the calculator will have included the cost for a 480 sq ft unfinished basement.

You can pick and choose items on the cost breakdown that apply to your project. Converting the unfinished basement to a garage will cost you a bit more.

I came up with an estimated cost to build of $103,000, including a General Contractor Markup of $11,774.

Keep in mind this is a “rough” estimate. Also keep in mind that cost per sq. ft. is higher for small homes (i.e. additions).

Good luck, Carl Heldmann

**Caution:** If you do put the garage under the addition, be sure you comply with local (as well as national) building codes as regards to fire proofing. You will have to at least have “fire stops” between the ceiling joists (floor joists of the addition above) and plan on drywall the ceiling.