Have a look at what neighbouring properties have had done. Also use the internet, apart from the planning portal most local authorities have a planning department on line where you can look at planning permission applications - by road. Obviously it may be that for a single storey extension you might not need planning permission but its quite useful to see what applications have been put in by other properties on the road and whether they've been granted. Some councils will also have a person within the planning department that can give you guidance over the phone (although I know our council has stopped this now).
flow4 is right, Planning Portal is a really useful tool. It tells you the rules about needing PP, and also has a handy graphic showing what this really means. We used it before booking an architect to design our extension. As we are detached and not in a conservation area, and were doing a single storey rear extension, it turne out that we didn't need PP. In most cases if you have a detached house and are doing a rear single storey extension that is less than 4 m deep and no wider than the back of your house, then you won't need PP. But you do need to check as there are different rules in conservation areas, or if the property has already been extended, or if the new extension is going to take up more than a certain percentage of garden. I was amazed that we didn't need PP for our new room, but really pleased as well as it speeded up the process not to mention the cost. You still need Building Control approval though.
Hello Trying to buy a house. Most places we see need some external work i.e. single storey extension. Is there a quick way to find out whether they would be likely to get approval from the planning bods? All I can find is details on whether you need to apply or not, not what sort of criteria they look at and factors for making proposals successful or not. Cheers