Need someone to build a loft room

[Desert rose]

Basically the house is semi-detached and flat-roofed and we are thinking of adding a room on top in the middle since the house is quite long. Or should I say deep. What we are looking for is accompany that can draw up plans, submit to the council and build. We are looking at timber construction. If anyone can recommend a reliable and honest company, that would be most appreciated. We are in Cape Town.