You can use your tax refund to pay for all or part of your down payment on a home although there are a couple of points to keep in mind. First, if you currently do not have sufficient funds in your bank account and you intend to use your tax refund to pay for your down payment, closing costs or reserves then lenders require that those funds are deposited in your account prior to closing your mortgage.
Most lenders will accept your loan application before you receive your tax refund but make the receipt of the refund proceeds and the deposit of those funds into your bank account a condition to closing your mortgage. When you apply for the mortgage you should also provide the lender documentation that verifies the amount of your tax refund and the expected timing of the refund. In short, you can shop for homes and lenders and apply for your mortgage before you receive your tax refund as long as you are confident that you will receive your refund prior to closing your mortgage and home purchase.