If you have a borrower puchasing an investment property, do you calculate the Rate Spread?

If the loan is subject to Reg Z, you need to do the rate spread calculation.

Investment property would be considered business, so that would mean no spread calculation.

That is not necessarily true. Personal investment loans are not exempt from Reg. Z.

We are getting from our borrower a signed business purpose statement in those cases.

A signed business purpose statement alone will not suffice to determine the loan's purpose. The lender is responsible for determining the loan's purpose by applying all the factors found in the Commentary to 226.3. A signed business purpose statement is only one of the factors.

What if we coded and classified the loan as commercial/business and we know it will be used as a rental house etc - then in our eyes it would be a commercial loan/business purpose and not subject to Reg Z - we are doing this bank-wide - is this not correct?

The acquisition, maintenance and rehabilitation of non-owner occupied rental property is exempt from Reg Z. Therefore exempt from the rate spread reporting requirement.