1. Don't wait. Get planning permission and engage a builder asap.
2. A kitchen is for life and its contents will have to last for many years. Don't cock it up. Most likely you are your own Project Manager, ensure you have an easy time.
3. If you can find a builder that is good, considerate. careful, neat, helpful etc and will get the job done to your expectations or even exceed your expectations love him, cherish him and pay him.
4. Believe me the last thing you need is a bad or inconsiderate builder.
I've just finished a kitchen extension, found such a builder! He was really great, I've hired a lot of cowboys over the years but did well with this one.
My extension is way smaller than yours so not much good comparing costwise. I also didn't have much option shopping around for the actual kitchen as the extension started last Oct but come January lockdown I couldn't even get a kitchen supplier to put me down for consultation until this Sept so went with only one who had availability. Again I hit lucky and got what I wanted at a price I considered reasonable but it's a how long is a piece of string question regarding a kitchen cost, a person could easily spend double/treble what I did on the same kitchen depending on your tastes and budget. Luckily I have simple tastes!
There are some horror stories at the moment around small extension jobs and the costs escalating especially around the cost of steel if you need it in the extension. Apparently builders are really struggling to source the steel and the price is changing daily and can change dramatically between from when the job is initially priced to the delivery date. I know one builder doing an extension who lost his supply of steel on the day it was supposed to be delivered because another builder offered the supplier double the price. As above, find a good builder and make sure you have a healthy contingency budget.