Tax Implications of buying and doing up and possibly renting back??

Part 1
We are currently purchasing a house with a view to renovating and then moving in, we are not selling our current house immediately. I assume we have no CGT as our current house remains our PPR.

Part 2
The current owners have asked us to rent the property back to them for a short period after we close, this may suit us if we require planning permission. I assume we would have to pay Tax on that income or is there a minimum threshold we can stay under? Are there any other implications of this transaction?

thanks