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We did a double extension, but extra costs were all due to us choosing to extra stuff.
1- all new upstairs windows and render to make everything match.
2- new lead work on chimney as it needed doing when new roof parts went on.
3- new landscaping outside
4- a lot of extra electrics and nice fittings )new consumer unit.
5- a new boiler and moved stop tap.
6- had posh rads rather than basic
7- flooring and fitting is more expensive than I realised!
Tbh our builder was great at keeping in budget and doing things to budget.
Our kitchen came in on budget (DIY kitchens). and bathroom we sourced everything ourselves so saved over 1.3K just on materials than buying from a kitchen place.
Good Luck! I reckon we added easily 20K on top to complete finish.