When you build a home addition, what does a quote "per square foot" include?

In general, additions will never make the home more valuable on the market. Additions cost more than they are worth. A quote for cost per sq ft should include everything including the foundation just like the cost per sq ft to build the original house. That includes the building permit, foundation, site preparation, and all.

Additions break up the original architecture of the house, and often the additions are maladaptive. Meaning the addition causes loss of function in other areas. Some people will convert their attached garage to a family room or an additional bedroom. Then the car must be left out.

So, sometimes the house is worth less with the addition than with it. Houses are assigned a cost per sq ft ; and better quality houses cost more to build, and to buy. Additions will lower the quality of the house due to changes in the architecture which makes the entire house worth less per sq ft than before the addition. Usually not a good thing. Yet, if you never plan to sell the house the addition might be worth the convenience; if not, then you will lose lots of money.

'per square foot' estimates are usually general pricing that does include everything

once specific choices like siding, windows, flooring and other finish materials are made a more definitive price can be given

additions can be built to replicate the original architecture of the home, these are known as seamless additions and if you didn't know it was added on you would think it is original to the home