Second mortgage

Hi ,

I own a flat which I have been trying to sell but can't in the current climate.

My wife and I are living in a rented house short term while we find a new house to buy.

My question is if I try to rent out my flat does it matter if it is rented or not at the time of applying for a second mortgage, ie does it have a bearing on the affordability of the second mortgage ?

Thanks for any help