Another question is what is the practical effect of requiring an application and preapproval, but you did not ask that question.
Builders can require you to get pre-approved with their designated lender BUT they cannot require that you use them. Usually they will give you incentives and bonuses to use them in the form of "free" options and/or closings cost assistance but it really depends on the builder and how well they're selling.
I've been away from NOVA for so long now I am not quite sure how the Sterling/Loudoun market is...when it was slow in '07-'11 the builders would still give you incentives but allow you to use who you wanted to but even then they required that you get a cross approval through their lender. This protects them and makes sure you're truly qualified.
Good luck on your new home purchase!
If you are preapproved already with a lender that you are comfortable with, you need to have your agent tell the builder's rep that you are going to use the lender of your choice as that is the law. You do have the right to choose your lender. However, builders use certain lenders as they have a relationship with that lender, so they can sometimes give you more incentive money as that lender will offer it and the lender you were previously wanting to use do not offer the same incentive. Also, the builder is more comfortable using that "preferred lender" as they will fire that lender if a deal falls through, so that lender is going to be more diligent in getting the loan to close so that they do not lose that business or the relationship they have with the builder as they give them a pretty steady stream of business.
Compare offers, but if you do not want to work with the builder's preferred lender...you should not have to. RESPA laws state that you can pick your settlement provider and your lender...your choice. Not someone that the seller/builder wants you to use. Have your agent "remind" the builder's rep that you have a choice...period.
Good luck with your purchase.
Real Estate Consultant/REALTOR(r)
One word of caution: By allowing the builder to pre-approve you, you do reveal your personal and financial information to the builder's lender. Make sure, it is not used against you. Ideally, the other party, seller/builder, should NOT have access to the buyer's financial or personal information.