Side extension permitted development: the envelope sheet

The side extension permitted development envelope sheet reads what a house may gain without a planning application from your own measurements rather than from a rule of thumb: whether it is detached, the width of the original rear wall, the depth of the original side wall, the curtilage around the house excluding its footprint, the outbuildings and enclosures already standing, and whether the plot sits in a conservation area or other designated land. It returns the rear extension depth the order allows and the deeper depth available under the larger home extension route with prior approval, the side extension width as half the original house and zero on designated land, the floor areas each gives, the outbuilding area still available under the half-curtilage limit, and the ground floor area the plot may gain in total. It is the reading a developer makes at the viewing, with the arithmetic written down so the offer can be argued from the envelope rather than the particulars.

Why the sheet reads from the original house and never from the current one

Every limit in the householder classes is measured from the original house, the one first built or standing on the appointed day, so a house that has already been extended has already spent part of its envelope and the particulars will not say so. The sheet asks for the width and depth of the original for that reason, and a developer who measures the extended rear wall reads a depth the order will not give. The record keeps the original measurements against the plot with where they came from, so the reading survives the next viewing.

The side return is the envelope designated land takes away

The side extension is where most of the value in a wide plot sits and it is the class a conservation area, a national park, an area of outstanding natural beauty or an article 4 direction removes entirely, along with cladding and the two-storey rear. The sheet takes designated land as its own input and zeroes the side width when it is set, because a side return bought on the assumption of the standard envelope is the most common overpayment in this trade. The constraints map is the authority's; the sheet takes what you read on it.

The outbuilding room is a share of the curtilage, and the curtilage is measured

Outbuildings, sheds, containers and enclosures may cover no more than half the curtilage excluding the original house, and what already stands counts against that half. The sheet returns the room still available from the curtilage you measured less the outbuildings you counted, which is the figure that decides whether an annexe, a studio or a garden office scheme fits without an application. Curtilo Pro keeps the envelope reading against the plot with the measurements it was made from, so the next plot in the same authority is read from the last one's file; the order cited below is where every limit comes from.