drainge mods and rain water

I will be changing the drainage as part of a single-storey extension. Old hopper currently drains bathroom and rainwater via hopper down a pipe next to the soil stack, then into the sewer. Bathroom will be changed to drain into the stack. But, what to do with the rainwater ? The new roof will be in the way of the rainwater pipe, which I shall remove anyway. The new roof will have its own gutter, entering a gully I suppose. The question is, should I let the house roof rainwater drain onto the extension roof and into it's gutter, change the house gutters to drain directly into the extension gutter, or some other arrangement ? Are there regs concerning the amount/size of pipework to cope with the rainfall from a whole house ? Thanks, Simon.