So basically you're looking for a single room the size of a high school basketball court?

This is likely not a question to which you can crowd source the answer, as it's a very unusual request. My first thought is that it likely won't be the cost savings you expect due to the beams required to support such a space (unless you're okay with columns throughout?) Also, generally building things far outside the norm doesn't reduce costs. But only a local contractor can really tell you.

Hopefully, some current real estate appraisers will chime in, but as a former appraiser, I say that it's a terrible idea to have a 4,000 square foot bonus room attached to a 1,200 square foot dwelling.

I'm not really interested in appraising info.

I'm fine with columns.

You’ll be interested in appraisals when it won’t appraise at even close to the cost to build. Because that will mean you pay cash for everything. If you can even find any property anywhere that would allow such a setup. It would be better to think of such a structure as a barn or shop, and separate from the house. It might have a chance at approval then. But, you’ll be lucky to get a 10K bump in appraisal on a 300K outbuilding structure.

Define "empty room." Are conditioned? Heated? Finished walls? Flooring? Windows? Lighting? Storage? You'll need more specifics and to go to someone local to get a better idea of what a 5,200 square foot house is going to cost in your area.

I know people (car enthusiasts..) who set out to build big garages and small houses. Garage space generally has a lower cost per square foot, but here garages are not heated or cooled, have few windows, and otherwise have limited/cheap finishes.

Whatever it costs, you won't get it back when it comes time to resell, unless you are talking about something commercially zoned.

Lots of concerns over the appraisal. Not really interested in those opinions. I'm paying cash.

I actually answered all those questions in my above post: The room needs drywall, flooring, electric, heating/ac. That's all

People just want to make sure you are going in to something like this with eyes wide open. I'd look at costs to build a very large garage, or a commercial style building in your area to get a remote idea of what the cost might be. If you are serious about this, then talk to real, local people. Someone on here might have some inkling of costs for a remotely similar project in your area if you post your location, but even then, the internet is not going to be a good or reliable source for a cost estimate.

Gross square feet 4,000

$/square foot $125

subtotal $500,000

Contingency $125,000

Misc. fees $20,000

Approximate Rough Estimated cost $645,000

What do I win?

Mark, worst answer yet.

It doesn't take a genius to figure out what Hu182 is doing here. Small house, a nice building next to it, cash. C'mon folks. He's not going to the bank for this. And he's in the feasibility stage of this operation. Best to keep it quiet for now. Hu, I think you're on to something. Keep the ball rolling!

Cook's Kitchen, thanks so much for the great answer. Lots of good info!

Steve OnaRiver, exactly, thanks!

A big determinant of costs is siding, foundation and roof. A large expansive roof is not cheap. Obviously a thin slab is cheap - is your land flat? Are slabs ok with your soil?

Is your siding vinyl or stone. That is a huge variable.

Do you have permits and how strict is your jurisdiction. One I know of requires a bedroom septic for every 700 sqft of floor space - so you would need a 8 bedroom septic.

Are you looking for 8 foot ceilings in such a large space? I would expect at least 10 ft.

Flooring runs from $3 a sqft to $15. What type of flooring are you doing?

In my area of $150 for typical, you might get down to $50 a sqft - or you might be $90.

My estimate is based upon the information provided (+/- $200,000).

There sure are a lot of defensive, paranoid people on these forums.

"I found an experienced local contractor to give me a straight answer . . . "

Now you're getting it!

You finally asked someone that can more easily gather the information needed to give you a proper answer, instead of asking total strangers of varied backgrounds that you failed to tell where the project was even located. Construction costs are geographically dependent and every project is different.

I've decided to start down the road of building a custom home, but I'm looking for something specific. I'd like a large (4,000 sq ft) room attached to a small (1,200 sq ft) home. This would be just a bonus room, no bathroom or kitchen. Just floor, drywall, electric, and heating/air.

Thoughts:

- Will this be one big space, or will it be 4000 square feet divided between a two-story space and a basement? The look, function, and cost would be very different. This answer will affect your foundation and ceiling height too.

- If this is one big room, you may need some pillars for support in the middle -- how will that affect the function of your space?

- To what standard do you plan to finish the space? That is, will it be cinder block simple or finished with luxury materials? That'll be a huge difference in price.

- 4000 square feet is huge -- I can't imagine not needing bathroom facilities nearby. Is this going to be a commercial area?

- Even if you don't need bathrooms or a kitchen, will you need water bibs on the exterior?

How do you plan to use this space? It's hard to get our heads around the idea without this information.

I say that it's a terrible idea to have a 4,000 square foot bonus room attached to a 1,200 square foot dwelling.

I don't think you're going to get much disagreement on that. We don't yet know how the OP plans to use the space, but it's going to be essentially impossible to resell.

Lots of concerns over the appraisal. Not really interested in those opinions. I'm paying cash.

You don't know the cost, aren't sharing enough details that anyone could make a rational guess, but you know you're paying cash?

I know many dog breeders who have built just this. Both were not concerned with resale but one became so when zoning laws now limited her to 4 dogs and she had 29 plus boarders! It was very hard to sell.

We built a 3500 sf pole barn 2 years ago, to use as a maintenance building for our factory. 16' clear ceiling, concrete floors, 3 roll-up doors, lights and electrical outlets. No HVAC or insulation. It cost $165K, all hired out to commercial contractors. Midwest small town.

If the OP wants a barn, that's one thing, but if he wants a gigantic "bonus room" he's looking at a different level of construction: foundations, flooring, insulation, wiring, drywall, etc. And I can't imagine he won't need plumbing as well. Plus a massive roof with adequate support. Assuming he has some sort of commercial objective in mind, I hope he's factored in the costs of the entire build into whatever business model he's working on, because he needs to generate sufficient income to cover the cost of room plus house (since neither will have a resale value) before he can think about a profit. Personally, I'd build that separate barn or workshop. Separated, both it and the house would have value; together, not so much.

Mark Bichak - The point of a forum is to ask questions and converse with a group of total strangers who may have done something similar. And a couple people here have. Kind of a useless comment.

A 4,000 sq ft ranch (since it is all attached to the house and on the main floor) is going to be pretty expensive. The only thing you will be missing is the cabinetry and plumbing. You still have the very expensive items, foundation, walls, trusses, roof, etc. If you don't have support walls in the middle, then you need expensive supports to support the roof.

Since you are paying cash, you don't need to worry about financing. I'm going to assume you aren't accountable to an HOA, because none would ever allow this. So your next hurdle (other than paying $200,000 or more for this structure) is zoning. Call your zoning dept to see if this can even be done. My guess is your home will be assessed very high due to being a 5,200 square foot ranch. Resale will be non-existent.

The tip-off to anyone paying attention will be the the 1000 amp service that you’ll need. 3 phase is more efficient. But impossible to get in a lot of locations. Floor drains will be a really good idea too. Out in the boonies has to be a prerequisite. Some place that already has a motley assembly of large agricultural buildings together with not so large houses. They’re usually separate buildings though. And zoned for agricultural residential. Doing a hobby heirloom tomato patch could pay off in a couple of ways. A cash build is still gonna have to be reported to the IRS. So, there’s that.

I had a client that grew medical marijuana commercially in a pole barn, but he is smart and knows how to treat other people on and off the internet; unlike some people that find it necessary to attack others when they don't get their way and in doing so expose themselves for what they are. You, under the disguise of HU-956239182, will do better treating other kindly or ignoring comments you do not find helpful.

I may have missed it, but I can't see that you answered questions about exterior finish (siding vs brick etc.), type of roof, ceiling height, quality, size, and number of windows, type of flooring, whether the lot is flat, etc. All of these factors could affect the cost tremendously, in addition to many other factors I don't know enough to list.

What you want to do makes 'absolutely' NO SENSE....it sounds like you have 'more' money than 'common' sense. .........period!!!!!! The GC will LOVE making the money!!!!!! It will be 'very' cheap to build.

robin0919, what? You contradicted your self in addition to your normal diatribe...................................