We projected figure of 8% per year, but in reality it has been much higher increase.
I'm sure you are aware that each year group's fees are more even without taking into consideration the inflation rate.
Ours had a huge unexpected jump a few years ago to make up a teachers pension shortfall and again for the start of this year to bring the more in line with the boys prep school (on the same site, different school under the umbrella of one school) which irks me as the boy's day is longer and they get first option on all pitches, halls etc.
DD has also recently discovered to have a problem similar to dyspraxia (but not, more propreception) so we pay for all senco and extra help she gets.
DS is in p/t nursery (of a diff prep school) and has physical/learning disabilities and has to have a one to one which we pay for.
Bear in mind that it is not just the fees, our 'extras' every term can run into hundreds - insurance, school trips, theatre vising the school/school visiting theatre, going to farms, end of term picnic/party, visit from Father Christmas/Easter Bunny etc And we haven't even got to the PGL/skiing are yet.
Uniforms are generally much more expensive and there are many more items to buy - DD has to have, running trainers, indoor trainers, play trainers, football boots (for hockey and X-country) as well as school shoes. Although most independent schools have a good second hand shop, where the sensible one amongst us vie for the newest looking blazer.
TBH we could not have planned for every eventuality, no matter how good our accountant is (and he's ace!).
It's a bit like doing a huge restoration/building project on a property - you set your budget but allow a 20% contigency, which you know will get used up anyway.