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Worth noting you do use very cost effective builders, I wouldn't consider your costs average but on the cheaper end of the scale.
AGL Lofts are a great example. 40k is very optimistic price wise, and you'll be making massive fittings, finish and service sacrifices to achieve that price. Using a proper company run by adults ours came in at a shade under 70K I think with nothing special fitting or finish wise (hell, I'm having to paint it all).
Friends just had a small extension and loft done and it only just came in at 150K and tbh they used pretty cheap people (and the finish in most places is suitably bad).
The cost of the materials has not generally gone up much from a year ago. The cost of labour I'm not so sure about. I know you can still get a basic d ormer for 40k. My polish builder is quoting 55k for the extension I had done for 45k a year ago.
Unless both extensions are enormous and very high spec 110k should be a reasonable budget. But all depends on the spec really and how much the builder is marking up the materials.