So you want to buy a new house but you have to sell your house first?
You have few options and here they are...

Option 1: Put an offer on the new home with a subject to the sale of your old house.
Pros: Safe, no risk.
Cons: You will have a tough time negotiating good terms for your new house since this subject is too complex.
The builder will still market the house you want and if the better offer comes in you will get bumped.

Option 2: You can trade your house if the builder is in financial position to do so. (Just like trading in your car.)
Pro: Clean, no hassle selling your old house, quick move-in.
Con: The builder won’t pay you the market value since he will have to sell your old house and there are some costs attached with that, and they do not like losing $$$.

Option 3: Compromise between option 1 and option 2
Negotiate with the builder to give you a guaranteed trade in price but you would reserve the right to try to sell it yourself in the next 60 days or so.
Pro: No risk. You are moving regardless of what will happen.
You can start packing. There is a chance that you will get more $$$ for your house than the builder would pay you.
Con: I can’t think of any.

I hope this will help you.

Tibor