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The rest of the list (rewiring throughout, new fixtures and fittings throughout, repair / replace the facias, possible work to the roof, anything else obvious) has huge error bars depending on what’s needed. Could be £5k but could be £50k. There’s other stuff not on there (boiler?) that could add up as well if it’s the same age as the rest.
For example, when you say “rewiring throughout” is that a spark’s view or is it based on eyeballing the plug sockets and CU and thinking it all looks a bit old?
Leaving the existing rear wall in place would help massively on budget and there is a question of really how big any one room needs to be. If you build across the whole back including where the WC is you’d have a (?) 25 sqm room which I think is more than big enough.
Doable with some change from £100k depending on how fancy you get with cabinets, appliances and glass roof / bifold / crittal.