IRS has issued regulations outlining the responsibility of tax preparers regarding protection of client information.
Section 7216(a) provides for criminal penalties on tax return preparers who knowingly or recklessly make unauthorized disclosures or uses of information furnished to them to prepare a client’s return. The exception is when the taxpayer provides the tax preparer with written consent to the disclosure. The taxpayer must knowingly and voluntarily provide consent.
If you will need any tax information forwarded to a third party you will need to sign a consent statement that has the names of the taxpayers and the tax return preparer, the purpose for the disclosure (obtaining a mortgage), and the tax information that may be disclosed. I have the forms if you need them.