The larger SF's typically procure less per SF. Calculate each, based on the closest comparables - condition, style, lot and size. 

I see agents all the time who calculate their price based only on PPSF and that just doesn't fly when it comes to the appraisal.

I rarely use price per sqft to calculate values as it leaves so much of the appraisal formula out.

You'd have to categorize the comps first by location, condition,amenities, then do the avg. $ per sqft for each group, then apply that to the subject property as appropriate...given the info provided.

Do people really buy by the square foot, or do they buy the whole package?

($167/sf gla * 2501 sf gla)*(3500 or 1500/2501)= Blah blah

That is basically how most AVM works...

Assuming there is no adjustment on residual acrerage variance, often $1-2 /sf  outside coasts. 

  For Florida: go 60 days out and adjust for recent capital improvements over and above the $/SF 

Size of homes taken +/-500sf to subject home and age =/- 5 years try  for same # bedrooms and baths. we adjust $20k on bedrooms and $25k on baths.

Do not be afraid to deduct for deficiencies

To go strictly by raw $/sf and averages is doing your client a disservice, and ultimately you in the end.

Square footage and $PSF are the least weighted attributes in my property valuations. That being said I enjoy a +/- variation of 2% for LP to SP. There's not nearly enough information here to shoot out a number from the hip.

As Thomas J. Nelson, Realtor,CRS,e-Pro commented, we are both in CA although he in Southern CA and I am in Northern CA, inasmuch as cost per square foot goes, it rarely, if ever applies in my market.  

Sam Shueh who is in my market has provided the mathematical calculation.

Not enough information. Incomplete data leads to poor results.

I would use comps of approximately the same size.

SF only applies any cookie-cutter subdivisions with lots of recent sales.

I wouldn't do the sq footage method at all. You don't say what the subject is but I would use a comp closest to that & then you'd still have to get a backup for that one.

More info.

I only use the price per square feet between comparable properties.

If it will be only only difference it would not be that difficult, but I'm sure condition, location, pool/no pool, view/no view, etc. etc. information is also very relevant. That's why agent estimate and Z.( that rely mostly on sqft) can be so different. 

Ditto.. Debe Maxwell, CRS !!

Inaccurate information posting is a problem with all MLS's. When doing CMA's I evaluate by similar homes and do not place as much emphasis on sq. ft. prices.

I don't come up with sale price by by square footage - sure it's consideration but not on top of the list. There is so much variance between homes in every neighborhood especially in my market.

I do not use a price per squre feet for residential homes. Too many variables such as lot value, extras, upgrades, condition and such. I feel what has sold is much more valuable to using in my valuation

Look at the entire city, not just a 1/2 mile radius.  Then adjust for newer/older home and nicer/worse area.

I do marketing....my formula reflects the number of titles I have on the SERP for a large variety of keyword phrases relative to my marketplace for listing....

I do not do Square Footage 

Sam Shueh gave you a great answer. A

I do not use it

Joe Pryor nailed this one!

Not via price per square foot... which doesn't take into account land, upgrades, number of rooms, etc. Price per square foot for me is simply a last check to make sure the numbers are not totally out of the realm of reality.

Using only hard numbers is not the way I value a property

Annette Lawrence , Palm Harbor, FL 727-420-4041 - our MLS has 'adjustment' feature for a difference in square feet - multiplied by $/sq ft as your adjustment.

Well I guess I just wouldn't do it that way as my first stab (assuming condition/location/acreage is abt same) would be # of rooms and layout (where is the extra square footage...each room larger, sq footage in basment (clearly lower value), larger kitchen, etc.

Then, after looking at that (if they are =) then I might gross up per sf, but prob w/ a discount factor.  This is how things work in most industries - economies of scale.