For partial acquisitions, and when the dwelling is on a normal residential site for the area, reduce the mortgage balance and monthly payment to the same proportion that the acquisition bears to the whole property value before computing the increased interest payment. However, when the mortgage requires the mortgage to be paid off, treat it as though it is a whole acquisition.

Where a dwelling is located on a site larger than normal for residential use, it is a multi-use property, or the site has a higher and better use, reduce the existing mortgage balance and monthly payment to the same proportion the residential value of the dwelling portion bears to the before value of the whole property.