How do I bathroom / kitchen / extension? etc.

Here are the plans - we've not got to the full plans for planning but that is the next stage. Also no itemised costs but a high level view. The outlined costs for this work, they have estimated to be £280-320k all in which is 30% over what we had given as our budget. I know they've specced a ton of bespoke cabinetry etc which won't be cheap, so think there is a good amount of room to sharpen the pencil on this.