I built extension 16 x 9 for play room for kids, also got supporting wall taking down to make dining room and kictchen open plan, Plumber and sparks cost nothing ie me bro and his son, so all in cost 21K which included solid wooden floor layed in the extension and open plan room, 2 velux, used window we took out for extension window, the knocking down you should manage if anyway handy with a few friends to give you a hand, make sure you get the roll on skip cost around 900E but worth for what you are doing, I think if u know the right people it can be done for around what you are looking for, I was qouted 40K for my work and that included me looking after sparks and plumber, so I think do it but plan it out fully, someone will know a roofer, if u want number for guy who did the block work and slabing, plastering sent mess