You need to look on the council planning portal and see if other houses on the street have done the same extension already

Hi all,

I'd appreciate some help and realise it's general rather than specific.  I'm looking at buying a new property between the commons.  Some properties are listed as 'room for extension subject to planning', which usually means adding a room to the second floor at the back (running above the first-floor bedroom, which is in turn above the kitchen below it, with a side return having been completed already).  

I am just wondering what the chances are of buying a property with a view to extending this upper floor to add an additional room and then finding that the planning approval is denied, which I wouldn't OK with.  My intention would be to add a bedroom only and nothing outlandish in the design.

I guess I'm trying to understand how I make an informed choice, as I am not in a position to apply for planning prior to any purchase.  

Any advice is appreciated.  

Thanks in advance.