it depends what the extension is
if it includes a kitchen and a bathroom it will be nearer £45,000 – £50,000 (depending on the specification of course) including all fees and VAT
if it was say a dining room with a bedroom above i would estimate £35,000 – £40,000
assuming you are goung to get a bonafidi builder to do it all
if you were to "project manage" it yourself and use individual trades you could get it done much cheaper although it may take you much longer!
it depends on all sorts of other things like location, space (to build, deliveries etc) etc