It's better to look at mortgage lenders before houses. Once you choose a lender, you can apply for preapproval and get an official preapproval letter that states how much the lender intends to lend to you and locks in your interest rate. When you choose a home to buy, show your preapproval letter to the seller to show you're a serious candidate in good financial standing. Getting a preapproval letter can give you a leg up on the competition. You aren't married to a lender once you get a preapproval letter, though. You can apply for preapproval with multiple lenders to compare each company and make an informed decision. You'll only need to show one to a seller, though.