It's hard to say. Does your area require a certificate of habitability for a new house? You may check with local authorities about leaving the upstairs uninhabitable.

Nobody can really answer. California will be a lot different than North Dakota, won't it? Check local codes for how much can be "unfinished" ........: )

It depends.

Depends on your local zoning regs if you can do that, and how extensive of an area you want to leave unfinished and to what degree. Talk to your permitting office, consult your codes.

It will be easier (financially) to do it this way than to finish up a one story home and then add atop - but pricing will depend on LOCATION and other factors.

The space has to be designed to accept live weight instead of being a hive of nothing full of trusses. So you unless the plan has that as a possibility from the beginning, including the space eaten up by the staircase downstairs, it would cost selling that house and buying another. Or the cost of demo of the entire roof, and then adding a whole other 1200 single floor house on top of a single floor house, with the resultant space lost down stairs somewhere .

If it is space designed to be built out from the beginning? Including stairs, dormers, and an adequate electrical panel, That has its own added costs built into the original home. The home costs significantly more up front for the structural and everything else. Then finishing out the same amount of empty undrywalled square footage “just” costs 1/4-1/2 of what the downstairs costs. That’s assuming everything upstairs is to be the same as the lower level, with the structural, stairs, egress windows, plumbing rough outs, etc, already handled.

As live oak says- there's a lot more to it. More and more houses are designed and build with lace framework trusses in the "attic" making that space unusable in the future. And yes - there's a lot more to it even if you have the room - specifying for live loads is different than having the first floor ceiling joints just support drywall and fixtures.

But to me the key is that your roof framing is less likely to be built in such a way that will even allow any space up there.

We want to budget for this while we are planning the house. We are in NC.

A local contractor is your only hope of getting reliable information.

What does a 2400 sq foot house cost? Because that’s what you’re asking about building. Not a 1200 sf house.

We are looking at doing a 600 sq feet with the walls tilted with the roof.

If you want a habitable attic space, you will have you use either traditional stick framing for the roof, or specially designed roof trusses.

How much extra will it cost? Without a finished design posted here no one can tell you. Even your local contractors will have to give you a guess without a finished plan.

I suspect most local contractors will just be guessing at the cost even with a finished plan.

Yeah we are just in the planning stage on what we want. Would it cost an extra 20,000,50,000?

Well...OK...we can play along...

Calculate the floor area and multiply by $750/SF. That should cover it...or not!

If you plan this from the beginning, it isn't that much.

You are not adding siding or roofing - correct?

You of course have stairs and those are variable but assuming a budget build, $2k should cover it. Then you are at a few k for not using trusses, handling load etc.

Plumbing - figure $3k for rough in 1 bath. HVAC - $5k for another small unit.

So $15k might do it in rural NC.

It is very common in NC to have unfinished attic space with walk up stairs so I don't think you have any concerns on the permitting side.

What you are planning is typically called a 1.5 story which is often the cheapest (ie most cost efficient) way to build.

Remember of course that you lose 1st floor space with the stairs (and 2nd floor space). That eats into the cost efficiency of the project on a smaller house. If you had 2,000 1st floor and 1,000 second floor, the space lost to stairs is less of an issue.

People hate cost questions here because there are a lot of variable and no one can give you a real answer.

You need to leave room on your panel and build a big conduit to this space. You need to figure windows which I ignored.

What are your long term goals? Not telling tax collector so doing it after CO and not permitting it?

Thanks! price questions im sure are interesting. We are Just scared to plan something and then go over budget. Most contractors around here won’t sit down with you and just answer questions. We tried modular but it was not a good experience. Nah just to save money during the build. It will be kids space/bedroom but we don’t have kids yet So there’s not a need to have it done right away.

I would just be sure waiting is legal...and finish it when the need arrives....if it never does ....many buyers love unfinished spaces that will convert to extra sq.ft that they dont have to pay for...

You'll get a better idea of cost if you plan to build an 1800 square foot house and ask how much you will save by not putting in the flooring and lighting fixtures upstairs. That's all that you mentioned in your original post so I assume you're going to frame up the walls and drywall, and there will be no upstairs bathroom.

One thing to consider. When you do staged construction, you need to insulate each compartment separately which is needless double work. You don't have to do this but the default would probably be to do that. You certainly will need a sealed door to upstairs that you would not need if you just finished on day one. Don't forget to get drywall delivered if you do staged. Much easier and cheaper if done with the main part of the house.