this thread is 100% guilty of assuming everything starts at circa 10k
my mum had a 2x2m hallway/entrance extension including roof, windows, all electrics, plastering, only thing not included was the front door for 16k, if you'd asked before in here before starting you'd have put aside 50k!
Dont forget this is the London £40k Kitchen and Single Speed Forum.
Easily doable for under £5k if you are OK not having top of the line everything.