What is the approximate cost of a loft conversion?

That will depend on your location but you could be looking at between 20-30k and of course there are building regs to think about and you may or may not require planning depending on the type of plan you have and there are fire regs to follow also have a look at the link below where some Q's are answered with regards to loft conversions ok.Also the second link for planning portal is the uk government site and clicking on there will give info also on your project ok,good luck.