How to achieve 260cms ceiling height with flat roof in my extension

It all depends on whether you are trying to build the extension under permitted development. I assume you are as you are talking different roof designs. Under permitted development you eaves of the roof should not exceed 3m (taken as a measurement from the ground level by the original house but at the extent of the extension, where the final wall meets the roof - not including any overhang so that should also be considered. TRADA span tables using C24 timber show that a 63 x 195 should be sufficient for the span (that depends on superimposed loads you expect) so a warm roof would take you up circa a further 155mm giving an o/a min height of 2945mm which means you can use 50mm firing pieces to give fall. If you fit a cold roof you can shave off a little roof thickness as some of the insulation is set within the roof and some under, rather than the full thickness all above. If you have no window obstructions above, you could fit a pent roof so the eaves meet permitted development criteria, but the internal flat roof may be higher, giving your builders a little more leeway for error. The ridges on the pitched parts of the pent roof should not exceed 4m o/a above g/l.