How much space do you lose building with a ICF vs 2x6 stick frame construction?

I was looking in ICF to build a 3000 sq ft house and one disadvantage I saw that deterred people away from building a small house with ICF is that it takes a lot of space. How much space does it use up? I saw some had various sizes like 4" , 6", 12" etc. Which are typical used to build a house with?

Odd question but I'll bite. Assuming the same interior finish and also assuming that the exterior does not calculate in to the total square footage..a 2 x 6 stick frame is 5.5" wide and a 6" ICF form is a full 6" + 2.5" of interior insulation. So.. you lose 3" on the interior. Assuming a 60' x 50' perimeter (3,000 SF) you would lose about 220 LF x 3" = 55 SF.

6" ICF is probably the most common size used for home construction.

My house is 2037 sq ft inside the walls and 2243 outside the walls. That means there is 236 sq ft in the walls. My ICF walls are 11", 6" + 2*2.5". If you figure 1/2" sheet rock inside both types of walls, and 1/2" sheathing on the outside of the 2 x 6 wall, then the ratio of the wall area is 6.5/11.5 and the additional for ICF would be 5.0/11.5 x 236 or about 103 sq ft. You can easily lose that much by poor floor design or by constraints caused by getting the floor layout you want.

In reality there probably won't be any significant loss, and maybe even a gain, by using ICF. That is because the 2 x 6 wall will be built inside the perimeter of 4' x 8' sheets of subfloor, and the ICF will most likely be built outside the perimeter of 4 x 8 subfloor sheets.

Yes, you lose some space, but it's really insignificant unless you are strictly constrained on the outside dimensions. Design the house from the inside out to get the area you want/need. I was able to successfully argue with the assessor that since you can't live in the foot-thick walls, you can't tax them as living area. They agreed, and based my taxes on the interior dimensions. I wouldn't consider a 2X6 wall to be efficient without some external foam to break up thermal transfer for the studs, so the difference becomes even less.

Typically, you will incur loss only on the outside walls unless you will have ICF walls on the inside which is not common. But it only makes sense to talk about loss of floor space if you are constrained by an odd size lot or local bylaws to stay within certain limits. If you don't have these constraints then there is no "loss" per se as you can just design the house a bit bigger.

I tell everyone 10". If outside of foam is outside of wall. ICF plus drywall is 11 3/4", 2 X 6 plus drywall is 6 5/8 if you are using 1/2" OSB. That's about 5" per side. I also tell them we will keep the "important" dimensions good , baths , kitchen, hallways and lose the rest in less consequential areas.

I've had a number of people ask me this question. If you do the math, sure, you lose some space. I see this as a concern if the building lot is very small, or if square footage-based taxes are very high. I successfully argued that since I can't live in my walls, my 'heated square footage' should be measured on the inside instead of the outside. Believe it or not, the tax office agreed! I'm sure I save more than enough in energy costs to pay for any additional taxes anyhow. Our house is quite small- 1295 sq ft inside. The comfort, quiet, and strength of ICF were well it worth it for us.