The primary method of showing a proposed new building or house extension is by means of what are generally referred to as plans. For a typical home extension this would include floor plans of any floor affected by the proposal. In the case of loft conversions this would generally include every floor in order to show the layout of any supporting structure below and also to verify that there is an adequate route out of the building (usually referred to as a means of escape) in case of a fire.
The criteria for this becomes more onerous at second floor level and above (see Loft Conversions and Building Regulations). This would indicate the use of the rooms and positions of doors, windows and any fixed items such as stairs, sanitary ware etc. At the Town Planning stage this may be largely all but the Building Regulations and for construction purposes it will generally show other aspects such as the wall construction, direction of joist span and critical dimensions. There may sometimes be further details such as furniture layouts, electrical and radiator layouts and plans at other levels such as foundation and roof layouts although these will often not be essential for straightforward home extensions.