Of course there are many rules so talk to a broker/tax advisor.

Are there specific rules that you go by?

Putting investing off in your earlier years is never justifiable from a mathmatical perspective because of compound interest.

Overlays are extra rules that lenders put on top of Fannie/Freddie (and others) guidelines. 

So in their mind, there is potential for increasing rents and that justifies a higher sale value...

Go to county website for the rules and laws there.

(You should read The 10X Rule if you haven't).And maybe one day you'll be the retired real estate investor at a retirement home who has a killer development deal for the 90-year-old civil engineer who never quit taking action either.

Rule number one; you have to know what it is worth. 

Im not sure about all the rules so i thought Biggerpocket is the best place to ask . and do mortgage company really come to your property to check if you live there or not ?

As a general rule, you want to Assign the contract if the profit spread between the price you have it under contract for and the amount the end buyer is willing to pay is less than 10% or $10,000.What is a reasonable price to expect the end buyer to pay?