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How about not replacing existing windows for now?
Buying used or unused stuff from people that bought the wrong thing/need a new kitchen after 2 years might also help, but it's loads of work. But my new kitchen tap was 1/3 of lowest price online and is basically unused.
If you manage that with all the fittings that might help, but it's a lot of work.
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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.
Pretty reasonable detail. There's 60+ items individually costed, albeit not split for labour vs. materials. Electrical work, plastering and plumbing/heating only have one line each, but major areas of work are broken down so, for example, in the extension: blockwork, steels, floor joists, stud walls, creating a doorway from the main house, roofing etc. are all shown as separate lines.