A new en-suite bathroom requires Building Control approval and they will also check the structural integrity of the loft with the additional bathroom fixtures.
When the loft was converted, a structural engineer may have been involved in designing the floor. If so, I suggest that you engage the engineer again to check that the loft structure will carry the new en-suite bathroom loadings.
The plans that you produce for Building Control will be sufficient for a contractor to produce a quotation and to understand the required finish and position of fittings during construction.
If you intend to install a velux in the ensuite bathroom, you may require planning permission if it is on a side roof slope or your house is in a Conservation Area.
If new soil vent pipes are required, you may need to apply for planning permission.