Is it a custom house, or a spec house? Do you own the lot, or does the builder? When we have bought a spec house or had a house built from a builder's plan, we have either put $10,000-20,000 down at the time the contract is signed and/or paid out of pocket for any upgrades. The thought process for the builder is that he does not want to put a bunch of upgrades in a house and not be able to recover the cost if you back out. It depends on where you are in the process. If you are just picking colors and light fixtures, you will likely have to put less down than if you are making floor plan changes, picking out countertops and upgrading cabinets. Any earnest money/deposit you put down will come off the sales price at closing. If the house is ready for occupancy, the percent down should be more a function of your mortgage since the builder has already put the cost into the home and will recoup at closing. I think there are two issues - earnest money/deposit for the house and any changes, and actual percentage of the selling price to qualify for the mortgage.
If you own the lot and are building a custom house, he is going to want all the money (in a few bank drafts as construction proceeds). He can't sell the house without the land if you default. Then it is up to your mortgage company/bank to make it work with the construction loan.
I'm not sure what happens if you don't have the cash to put down as earnest money - I don't know if there is some way to get it (like a bridge loan?) then roll it into your mortgage.
Generally, if you put less than 20% down when you get your mortgage, you will have to pay mortgage insurance (I think that is still correct with all the changes in lending).
Hope this helps - I wasn't sure if you were asking about percentage a bank would require for a mortgage or money a builder would require for a house that is not completed.