@Silas Melson,
I agree with @Bruce Woodruff that a price per square foot approach is likely to be the least accurate and least useful.
I do think that having a contractor give you quotes is MUCH better, but often not completely practical or ideal either.
Often when buying properties you will be looking at and assessing a number of properties before making an offer. It may not be practical to have a contractor look at each property when you might examine 10-20 for each one you buy. You also may need to work quickly to make offers and bring a deal together.
Also, a contractor may not be the best person to develop your scope of work! If you are doing a BRRRR the rehab for a rental you intend to hold might be different than a flip you intend to sell. I would say that most contractors would be good to collaborate with on scope of work decisions but many/most would not be ideal driving that bus on their own.
You also don't want someone with a dog in the race to be making decisions for your investment. The contractor is making money based on the work you hire them for. So, there is a conflict with blindly using their opinion to decide what work should be done. YOU are the expert in the kind of business you are trying to do. So, YOU or a project manager or a designer or a partner would be more ideal to have control over the scope of work.
What I do is develop my own scope of work and then make my own estimate for each line item. However, since I am NOT an expert in everything I assign a value for how confident I am in each estimate and use that to generate a LOW and HIGH estimate value. Those things factor in the variance needed to have confidence in my estimating.
For example:
Item Estimate Confidence Low High
Demo $2,000 90% $1,800 $2,200
Construction $15,000 80% $12,000 $18,000
Misc $4,000 75% $3,000 $5,000
----------------------------------------------------------------------------
TOTAL $21,000 $16,800 $25,200
Obvious this is a grossly oversimplified example. There will be MANY more items to estimate in a real scope of work.
If your "estimate" works for you and your "high estimate" is still workable for you, then you can feel confident that if you use good project management practices that you can manage the project to somewhere between those 2 numbers at worst.
Estimates are ESTIMATES! So, it will still be dependent on you to make MANY decisions that bring the project to a successful conclusion. For example, when something unexpected comes up and your costs risk getting out of hand, you might change the scope of work a little. Perhaps refinish existing wood floors rather than install new LVP to save money. Or you might choose to hire a handyman to do some work and manage them more closely than you would a more proficient contractor. Many decisions can be made to keep a project on track as it goes.
