Advice needed in whether to join two titles

We have just purchased the property next door to us that is an adjoining property. It consists of three terraced cottages. The fellow that owned them had started work to make them in to one property and then sadly passed away. They are presently under one street number. We would like to join the two cottages closest to us to our property to give us more space. The last cottage we would like to make into a gite down the track as a future project. Its totally gutted at this stage so not liveable at all. Can any of you tell me whether it is worth changing the titles by making those two cottages part of our deed and keeping the last cottage separate? Will this be better for any of the taxes we pay? And, should we never have the funds to do the gite project would it also help if we needed to sell that cottage?

I guess it could avoid any issues about maison secondaire…