My friends Sarah and Giles bought a property that had originally been designed from custom home plans. It had within its property a very old central house and an equally aged tiny house. When they first told me how they planned to rent out the smaller of the two structures while the renovated the main one, I gently hook my head in disbelief.
While the extra income from a rental property can provide a much needed cushion during a renovation, there are other things to consider. This I especially true if the buildings are situated near one another, and the repair are to be extensive. One of the main reason for concern I the consideration of comfort for the renters.
You should expect the construction workers and repair people to be loud and intrusive with their presence. No matter what is agreed upon in advance, there will be a mess left behind if not frequently, then at least upon occasion.