Is there a reason why you don't think it will cost $110 per square foot? It sounds like it will have the same things the other rooms in the house have. We are not finishing out most of our basement so that part will be a lot cheaper but the 800 square feet we are finishing down there will be the same per sq ft as the rest of the house.

Good luck! Always exciting to build a new home!

I guess the thinking is that because you've already constructed a garage and put up walls and would already have a roof that the cost of finishing out the bonus room would be minimal compared to a room in the house that needs it's own concrete foundation, etc. I'm sure I'm completely off base.

You're not off base. Building a two story is cheaper per sq. ft. than building a one story. When you were giving the $110 per sq. ft. price did the builder know you were building a bonus room? They may have considered it the the price per sq. ft.
The price would be 2800 + 900 x $110 = $407,000

I would verify with the builder.

You also have add the cost of a stairway.... I don't think it would be any cheaper, at least in my experience.

SF pricing is based on a average for all of the spaces in a typical house. Most of the cost of a bonus room is paid for by the space below; that is why it is called a bonus room. Extrapolating to predict the cost a a special space that is part of the garage will be a wild guess.

You feel like guessing instead doing an actual cost estimate for the house you might add 50% or more to the cost of the garage.

If you use this method carry a large contingency fund in the estimate and reduce it only when you get real pricing.

Be sure that the room is engineered for live loads rather than just storage. That adds to the cost. Be sure to add extra insulation detailed properly. Thes rooms are often cold/hot because this is done incorrectly and it adds cost to do it correctly over standard wish and a prayer that it will be all right. Be sure that the HVAC calculations are done correctly for the additional space. 900 feet isn't a trivial amount to add to your heating and cooling load. It could be a whole other complete system if you want the most control over a lightly used space, or it could be zoned from the the upstairs unit that gets bumped up in size. It won't be your basic budget choice though.

A bonus room only has the basic structure taken care of in its building, not the stuff that makes it actually usable. Sometimes it doesnt even include drywall or insulation as it is intended as storage space. So, you actually have more expenses per square foot to change that to habitable space thanan already existing bedroom for instance.

Alan

So we just finished building our home last year and our master bedroom is over the 3 car garage and the cost per square foot is about the same. You cant enter our room through the garage only the main house but as others have said, we had extra insulation in the garage ceilings and in the bonus room to combat the too hot/cold issue. In addition, we had venting run to the master bedroom obviously but also the master closet etc, and then windows on two sides of the wall. All in all, the cost is about the same IMHO.
If you need any more information, let me know.

yep, it is much less expensive to build over the garage. Ours is around 60/sq foot (compared to 200/sq foot on the first floor). BUT I don't know how you will get 900 sq feet of interior space over a 3 car garage unless it is huge! For example our 3 car garage is 38 x 25 and we are getting a max of 630-ish sq feet over the garage. A similar home by our builder had a 4 car garage and they could only get 720 sq feet of bonus space. And beware of the angled ceilings inside the space (not sure of the technical term for it) due to the way the space fits within the roof of the garage.

these bonus rooms are difficult to heat/cool
foam insulating the roofline helps to make
these areas possible to condition.

building up is cheaper than out.
have all elec, plumbing & sheetrock installed
during build to lower costs.

best of luck.

If total cost is $110, then it would be about $20 to finish otherwise framed space.

It is similar to finishing an unfinished basement if this area was planned as a "finishable" space.

I am assuming that roofline is there and stairs are already there, and subfloor is already there (and designed for load).

The real cost variable is HVAC - if you finish it later, then your HVAC won't be sized right and you may need a unit just for that space which can add a big chunk.

Rough costs per sqft
electrical $4
Paint $2
Drywall $2
Insulation $2 (mild climate)
plumbing $0 - assuming no bathroom
HVAC $4
Carpet $2
Trim $2 (includes door)
Builder $2 (low to make numbers work...)

HVAC is quite variable - it can be less if you are just adding a duct or two and not changing the size of the original unit.

The comfort of a bonus room is climate and insulation and HVAC design dependent. Done terribly much of the time. I have a bedroom in a "bonus" area and it is just fine without spray foam. It also only has 1 duct and no return but it isn't 900 sqft either. The walkin closet in that area is absolutely frigid with no duct.

I agree with the others, figure the same $110/sf.
In order to achieve that sq ft you're going up with at least a 5' hip wall above the garage space which you wouldn't be doing if you weren't planning on Finishing the room. You need the structural support, the wiring, lighting, drywall, insulation, flooring, windows, and HVAC. To meet fire codes finishing the room over the garage also means you need to drywall the inside of your garage.
We are building now and my bonus room over the garage is a little over 850 sf. I was naive enough going into this to think it wasn't going to cost us the same $/sf as the rest of the house.