That's something that we discussed and agreed was best, in our comments just above Bill's. You can leave the existing asset just as it is. Then add your MIL house as a new asset, and that new asset will be 100% business use. Like TGBill said, you'll only have a structure value on the new asset, with a land value of zero. That will give you two assets in the assets/depreciation section. One your main house with less than 100% business use, and the 2nd one the new contruction with 100% business use.
