Hi there, u can look for those freelance designer to help u to do the 3d drawing and all the detail drawing for your carpenter to do work.... if u engage id or contrator is must more easy for you, they will handle everything on site for u..if u can get a reliable carpenter u no need go for id...might scared u don have experience on the carpentary work details... Some id will provide you free design n drawings, and you no need to hectic to laise with your carpenter your own..but for your case i think u no need to find a contractor or id cos not much hectic work involved.... But u must have good n experince carpenter that u can trust...