Do you have separate costs for the construction? If so, just treat it as a completely separate rental property and use the cost for the construction.
Is the ADU a separate building? Generally, a short-term term rental is usually depreciated over 39 years (long-term rental buildings are usually depreciated over 27.5 years; you may need to tell TurboTax it is a Commercial property to get it to use 39 years). If it is the same building as the long-term rental, it gets a bit more tricky and perhaps the entire building may need to be depreciated over 39 years.
