Estimating vs. Basic Algebra (don't be scared!)

[EricVonHa]

A good buddy of mine built a nice 3 car garage a couple years ago. He was incredibly detailed with his expenditures and gave me a 2 page Excel summary detailing concrete, lumber, roofing... all the way down to the nails, caulk and door lock hardware.

Here's the question: If the square footage of my future garage is going to be 2 times his square footage... is this a straight multiplier??

(Let's assume for the ease of discussion that the construction materials and type of building will be similar)

750 sq ft = $20k

1500 sq ft = $40k yay or nay?

 

[logical]

I'd say it would be close but even more accurate if you'd double things like roof and floor costs but scale wall-related stuff by a factor of only 1.5 or so. (if he was 25 x 30, that's 110 feet of wall...if you are 30 x 50, that's 160 feet of wall...and not twice as much). For doors, you may or may not have more than him and same for mechanical stuff like wiring.

Oh yeah...do all that then add 5-8% for every year since he built.

 

[PAToyota]

Also consider that construction costs have gone up in the past couple of years because of everything from Hurricane Katrina to China's rapid growth to increased transportation costs due to fuel costs.

 

[shopking68]

Also consider that construction costs have gone up in the past couple of years because of everything from Hurricane Katrina to China's rapid growth to increased transportation costs due to fuel costs.

Thats what I was gonna say, well put.

 

[Rusty105]

I would think it would be a mix of both.
Permits usually go by the sqft. I would use sqft for the floor and Roof, so if you have double the sqft I would double those, for the walls do it like xxxxx mentioned. For features like doors, windows,lighting fixtures, and garage doors, I would use a per item method, if your friend has 2 windows and you have 2 use the same $ figure, if you have more, multiply appropriately. if you are heating or cooling the space I am not sure there is a linear way to estimate cost, might have to get an estimate for that

 

[unlvrebel]

Everyone has valid points, but your true question is would your method be acceptable. I think you have to ask yourself what variance you would be happy with. Using a straight multiplier would give you a rough idea and get you close, but "close" is a relative term - relative to the person using it.

If a variance of +/-$5,000 is acceptable to you, then a straight factor is fine. If that $10,000 margin of error is not acceptable then you need to get more detailed as the others here have outlined.