There are 9127 different variables that will impact pricing, with location being the #1 factor. The roughest estimates would have remodel square footage starting at double what new construction footage runs, but that doesn't take into account more than half of the variables. Remodels of a kitchen in an expensive urban area can easily be $2K per square foot whereas a family room extension in Iowa corn country could be $150 a square foot. Your project will likely be between those two extremes, depending on those 9127 varables.

You actually have to have plans to have local contractors bid on the same exact job to get any meaningful numbers. That means some skin in the game to get started. Hire an architect.

Thanks for the response, but I think we are on different pages. I am not asking the cost per square foot of a remodel or a house. In fact, I am specifically wondering about the marginal cost per square foot of building a new home. This means assuming that you don't add, change, or upgrade in any way the baths, kitchens, interior or architectual features, is there a cost factor for additional square footage?

It seems like there should be. I mean you are controlling for all the outlier costs, you are controlling for region, and controlling for design choices. I would think that someone in the building sciences has tried to put together the analysis. There may not be, the variables may just be too many but none of the things you mentioned are uncontrollable variables.

Building Cost Calculator

Again, we are not really on the same page. I found it, so no worries, some guys out of Wharton and MIT did it. Now I just have to dig through 50 pages and chase down 20 or 30 citations to find out some piece of mostly useless information.

It depends on where you want to add the additional area, and what material cost/labor rates are going for in your area. Are you building the home yourself, or have a GC? I am the owner builder for our home, and we added 600 SF to our large attic storage area for about $3,500.

EDIT TO CLARIFY:

During the framing stage, we converted what would've been vacant attic storage space into a 600 SF game room including a dormer window.

If you have a GC, expect to pay him much more than what it will actually cost him to add the area. Change orders are their bread and butter.

I am trying to do the same thing and GC are not really helpful. It is funny how when you ask them how much it cost to build the house everything else aside, builder grade they say well I can't do that because of variables. I think all people know that upgrade cost more, but if I give you a plan and ask for the cost a GC should be able to say how much they could build it for. I live in MD and from what I am finding 85-140 a square foot is about average for a 2500 sq ft house. Also, you can check NAHB they have the national averages, unfortunately I know MD is on the more expensive side.

I downloaded a few hundred pages to read, they are using a cost factor, in which they say something like, the cost of adding square footage to an existing structure that doesn't require an upgrade in materials costs a certain % of normalized square footage costs before the additional square footage. Normalized is cost per square foot of structure less cabinets, appliances and garage.

The problem with just finding the information is that they used it as a tool to get something else. Essentially, they developed the cost increase model so that they could model rates of new home sales in areas where existing homes were available on the market. I am sure they used someone else's research to get the model I just have to dig through the paper to find it.

Conceptually, you could take a cross section of a house and then price out materials and labor for every component of the structure, for it is the structure which envelopes the living space. For instance, you need one additional foot of footing, one additional foot of 8' foundation wall, one additional foot of sill plate, one additional foot of wall framing and sheathing, etc on up to the roof.

The problem, with regards to accuracy, is that widening a room one additional foot is going to impact on the joist spanning, if you have hydronic heating, your pex layout may already be maxed for the loop and so now you have to reconfigure your layout and so on, the downstream effects.

It shouldn't be too difficult to nail down the material costs for the structure and then apply some rule of thumb for labor to get you to your estimated marginal cost per square foot.

Another approach might be to pro rate the total of the foundation, framing, drywall, roofing, flooring, painting costs and exclude fixtures, plumbing, electrical, cabinets, windows, doors.

I agree with Alex. It would be simple to estimate additional cost given the budget/material details of a particular home, while having knowledge of material takeoffs and what labor costs are. But it sounds like you're looking for some type of numerical coefficient that would apply to any situation, and I am not sure that's possible.

I am not looking for myself, I am just generally curious. I see a lot of people throwing plans around who are going to extreme measures to avoid square footage, in my opinion, a lot of it is because the industry quotes square footage constantly. I looked at a few cost calculators that tell me the average cost in my area $99.50/square foot. Those calculators then take the square footage you enter and multiply by $99.50. We know that all square footage is not created equally.

The saddest thing is that the itemized cost calculators, such as the one linked above, are off even more. The cost it is returning for brickwork is 2.5 times my highest quote.

I am sure it is "possible" to do, doing it in a particular area would not really be that challenging. I mean the variables are not that esoteric and the data needed is available and more importantly free. If you data set is large enough (information on a few thousand would easily get you there,) the data gets normal pretty quick. It may turn out that once you got all this together your standard deviation is too large or the line has a bad fit, but even that would be published.

My assumption, and yes I know what they say about assumption, is that some academic somewhere has done this. I watched someone spend years looking at how the tax code effected the number of roof lines in English architecture. It was amazingly interesting for about 2 minutes.

The big issue is a labor cost. For the same job one contractor let's say installing drywall [2500 SQ FT HOUSE] for the total of 7600 sq ft of drywall will charge you $1900 another $19000. ($0.25/sq ft - going rate today vs $2.50 - absurd rate).

Interesting question.

I reduced square footage by approximately 10% in order to bring my build within budget. It turns out that there was virtually zero impact on the cost by reducing my square footage by approximately 300 feet. So I see what you are saying here. I would rather have the lost square footage at this point, because I seem to be paying for it anyway.

Just FYI...I did use one of those cost calculators during the design phase of our home. It might have been the one linked above...it looks familiar. I was surprised at the result. It was more than we were budgeting (at the time) and more than our architect and estimator were telling us our home would cost.

But in the end, it was dead on.

Another interesting but irrelevant observation: people on this forum quoting cost per square foot - you can tell who has and who hasn't actually built yet. The ones who haven't broken ground all keep saying they are getting all this stuff for $85 a square foot on one of the coasts. Heh. Good luck!

The other intangible variable is quality of the labor force. Sure, you might can find someone who will sand and finish site laid floors for $1 a square foot. But, would you really *want* to use someone like that? Good work costs more than shoddy work. When you use a compentent contractor, he generally weeds through the lowball incompetent subs. That's one nuance that many self GC's end up missing when they are calculating the "savings".

Construction is kinda like figure skating. There's the technical stuff, and then there's the "artistic" portion that everyone seems to have a different opinion about.

Hi,

Lots of folks use a cost/SF metric, but it's a really bad one and one that gets folks into trouble. Ditto for the cost to add an incremental SF. We liken it to asking "what's the cost per carat for a diamond?" There are simply too many variables which affect the metric.

For the homes we build, the only thing purchased on a SF basis is floor finish and even that depends on the specifics: For hardwood, is the hardwood laid perpendicular to the joists or is it on an angle? Is it in a herringbone pattern? Is there a contrasting border? Is the shoe mould included or not? Carpet waste will depend on the specific roll width, the geometry of the rooms and the direction in which the seams are run.

You will be best served to ask your builder about the incremental cost to implement a specific change to the base case plan.

The cost per carat of a diamond is easily established and in fact very public, and more importantly uses a factor just like I am talking about, you use a chart to select your stone's quality and then you multiply by the carats. It is called the Rapaport diamond report, it is updated weekly and available to anyone who purchases a subscription. You can always lay your hands on one.

I am aware that I think differently than most people, and really, I wish I had never brought this up. But people keep bringing things in as variables that are simply not variables. Again, the pattern of the hardwood will probably not change if your bedroom gets a foot bigger, most people would continue a herringbone pattern, they would only put a contrasting boarder if the rest of the room had one. Is shoe moulding included in the rest of the room or not? Carpet waste is a legitimate variable, but given a sample size of even 50 homes, carpet waste will get normal.

If you set the price of the rest of the square footage as the constant you don't need a number. By setting the overall cost per square foot to 1 (it doesn't matter the actual dollar amount) and testing for the relationship between marginal cost and nominal cost we get a number either less than or greater than one (a factor) that you can multiply overall cost by to get a marginal cost. With this method you don't need to really have two per square foot prices that are equal or even close.

I was working with a statistician today (on something else), and he also thought that someone has to have done this, it has a simple dependent variable and only a few independent variables. We found one study that quoted a factor of 0.58 with a standard deviation of 0.085. Which means that 68.2% of the time the marginal cost per square foot is between 49.5% and 66.5%, while 95% of the time the cost will be between 41% and 75%. I don't know where or when these came from so I don't think I would use them yet. I think I wil put in a call to the college of architecture at some point this week.

I don't think that issue is really marginal cost, it seems that the issue is more specific, marginal cost for structure only. For an individual home owner or contractor, the calculation for that marginal cost should be pretty straightforward, as per my earlier comment, sum the costs for structural elements and exclude the mechanicals, etc. and work from there. At this point the nitty-gritty cost accounting can be tailored. If you want a bit more square footage, you might be able to cantilever out over the foundation, and so avoid any increase in foundation costs. If the 1st floor cantilever occurs under an existing 2nd floor cantilever, then you save yourself added roofing costs.

The point we all understand is that an all-in sf cost includes things like windows, cabinets, plumbing, etc which are both expensive and are, for the most part, not a function of square footage. The key cost element in regards to simply adding more floor area are the structural costs. Isolate those costs, by costing or by estimating them as a proportion of mean total construction costs, and you have what you're looking for. The problem with the academic model here is that it aggregates all variety of residential construction and the mix between structure and mechanicals is going to be all over the map and so the error bars grow wider. When you're talking about your plan then you can get fairly precise in figuring out what it costs to add more empty space to the plan because you have actual bids for Version #1 sitting in front of you.

The paper that gave the 58% had an R-squared of .97. Which means that the line has a pretty solid fit (minimizes the error bars). I mean it is a wide variation, to be sure, but, still if these numbers are remotely up to date it could be informative. The jump in mechanicals would be part of the standard deviation (you have a 17% range in there and I would feel sure that the reason it is so high is mechanical and structual changes.)

I know I can do it for one house, I am not really interested in that. I just want to know. I will not be adding any square footage to my house, it is already too big. In fact we are trying to find places to take square footage out. Not because of cost, just because we don't want to have to clean that much house and if my house is too big my brother will find an excuse to take over my basement. The last time I built a big house my brother asked if he could move in for a bit until the divorce thing was finished. Nine years later I asked him if he had an ETA on the whole divorce thing getting finished...

On second thought, I thank all of you for participation in this thread, the input was valuable. At this point, I think I will bow to your wisdom and let this thing slide into yet another thing that I don't really need to know.

Frankly, I don't see the usefulness of the metric mainly because it was derived via a snapshot in time process. The issue of numbers being up to date doesn't address the dynamic of price movement with respect to the materials vs. labor ratio. As building trades become more characterized by Hispanic immigrants, the cost of labor becomes depressed so whatever the dynamic at point A in time, the effects of surplus labor availability are going to drive labor costs down when we look at Point B in time. Simultaneously, issues like inflation, economic growth and material availability are going to influence material costs. Labor and material costs don't move in unison and so whatever the ratio in one region or at one point of time, doesn't imply that the ratio holds across time. Granted, year to year differences will be slight, but they accumulate over time.

This is why building professionals seem to be expressing the position that "it depends" and that there really isn't a rule of thumb and that what rules of thumb do exist are mainly local.

I really appreciated your question because we are wondering whether to add square footage to our house plan, and I too thought adding it at the generic square footage rate would be an overestimation, perhaps. thanks.