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Kitchen can be a massive cost saving area, a lot of labour can be saved if you do it yourself, as well as savings on reducing spec on kitchen units, appliances and worktop.
Decorating, skirting, flooring/carpets/ could be done fairly easily.
Unless you’re really handy, probably get the doors done by someone else, door hanging, hinges, handles, routing and chiselling seems fiddly and time consuming, and easy to mess up.
Also I think you will need them in for building control sign off?
It's hard to overstate just how thermally awful the current (Crittall) windows are. We were finding ice on the inside of the bedroom windows over the winter. It's such an obvious win in terms of modernising the house that it seems perverse not to do it, especially since the scaffolding to get to the first-floor ones will already be in place.