Thanks for the link

It does say "A single story extension must not extend to over 3 metres from the original house to be a permitted development"

I take it that this means that if I want to extend beyond this - I need PP ?

Which, to me appears odd - since a garage of any size needs PP

Twee:

... are we going to have any problems in getting PP for such a large extension ?

You should be able to find the answer here...

http://www.planningportal.gov.uk/permission/

We are considering getting an (approx) 3 metre by 3 metre extension on the kitchen, at present - we have a conservatory of that size (which was fitted 8 years ago).

Obviously - it would be a waste to dissasemble the conservatory and chuck it out - so we are thinking about continuing the extension by moving the conservatory out to the end of the new extension

The conservatory did not require planning permission - due to it's size - but the new TOTAL length of both would be some 6 metres - which I understand may cause some planning problems ?

The area that we are talking about is about 50 feet away from a public highway and not overlooking any neighbours - so are we going to have any problems in getting PP for such a large extension ?