In most of Britain an extension design starts with the brief. In London it starts with the plot, because the plot has already ruled several options out. Boundary distances, the neighbour's windows, a conservation area designation, an article 4 direction and the width of the front door all narrow the field before anyone has drawn anything, and a design that ignores them gets redrawn.
What permitted development still allows
Householder permitted development rights survive in much of London and are the quickest route where they apply. They are also the rights most often removed in designated areas, and they never apply to flats or maisonettes at all. Read the technical guidance against your own property rather than against a neighbour's finished job, then get a lawful development certificate so the position is in writing when you come to sell.
House extensions north London: conservation areas and the northern boroughs
The Victorian and Edwardian suburbs north of the centre carry a high density of conservation area designations, which is why so many projects there go to application rather than certificate. The practical effect is on the roof, the glazing pattern and the materials at the rear, and on anything visible from the street. It is a longer route, not a closed one, and a well argued application on a rear elevation is usually determined on its merits.
House extension Richmond and the river side: its own questions
Along the Thames the extra layers are flood risk, ground conditions in the flood plain and a stock of larger houses in leafy settings where trees are protected. Flood zone status can change the floor level, the construction of the lowest storey and the drainage strategy. Tree constraints can change the foundation type entirely, from a trench fill to a piled or reinforced design near a protected root zone.
House extensions Essex and the counties: wider plots, clay and local policy
Wider plots, more side access and fewer party wall notices make the construction straightforward. The variables move to ground, where shrinkable clay is common and foundation depths near trees become the cost driver, and to local design policy, which varies considerably between districts. The same drawings will usually be built faster and consented differently.
House extensions Richmond, and what the borough changes
Richmond upon Thames carries some of the heaviest designation of any outer London borough: extensive conservation areas, a large number of listed buildings, article 4 directions in several places and, along the river, flood zone constraints that reach the foundations and the floor level rather than the elevation. The practical effect is that the permitted development assumptions this page describes for London generally are the least safe here, and an application is more likely than not. Establish designation and flood zone on the council's own mapping before a designer is engaged, because both change what can be built rather than only what it costs.
Questions people ask about house extension London
Can a flat be extended under permitted development?
No. Householder permitted development rights apply to houses, not to flats or maisonettes, so any extension to a flat needs an application.
Is a lawful development certificate worth the fee?
Usually yes. It is the only way to hold written confirmation that the work was lawful, and a buyer's solicitor will ask for it.
Why do foundations cost more near trees?
Because shrinkable clay moves as the tree draws water, so the foundation has to go deeper or be designed to resist the movement.
Does an article 4 direction mean permission will be refused?
No. It removes the automatic right, so the same proposal is judged on its merits through an application.