Before school holidays became a problem, we always holidayed in Cornwall during September. It is still a very busy month but no where near as busy as in August!
Lots of people wait for the schools to go back before they go on holiday - parents of young children, and many people with no children. Lots of people have a second holiday in September too.
The later in September you plan to go, the more likely there is to have a last minute choice of places to stay. But the first couple of weeks will be busier.
If location, sea views or quality of apartment is important to you then book now but if price is more important to you and you don't mind where you are or what facilities the cottage/apartment has then wait until you are ready to go and barter!
we have been to cornwall at all times of the year and the weather is so variable it is a bit impossible to predict. if you do leave it last minute then it is a risk that all the decent places have been booked. would suggest you have a look at a few places now to see what prices are like and perhaps make contact with them. ime sept in cornwall can be pretty busy as lots of people wait til the schools have gone back before going away. have a look at the cornwall tourist board website for places that have been visited independently for quality. we have stayed Here you might find that replies are few on this as its a bit like asking how long is a piece of string. if there is a blazing hot summer then likely cornwall will be booked out solid but if the weather is rubbish then there will be more spaces.
Ok, so I want to go with Dh and DS (who will be 18mo) to Cornwall in September as soon as the schools have gone back. My dilemma is that both DH and I are self employed (or will be by then), so we have the luxury of booking very last minute and changing our minds if the weather looks terrible and going a week or two later. This also has the benefit of saving money as it's likely we'd get more of a deal - we'd like a small self catering property but with decent facilities - particularly lounge with a 'nice' TV as it won't be much of a break for us if we're squinting at an old 14' trying to watch films in the evening.
However, I can't find any old threads on going to Cornwall in September and what the availability is like. There is still late availability for May/June which I take as a good sign that not everything gets booked up all of the time, but does anyone have any experience of this or have any advice? Have you been to Cornwall in September?