Three metres appears so often at the back of British houses for two reasons. It is roughly where a kitchen stops being a corridor and starts holding a table, and it sits within the depth that permitted development allows for many attached houses without the deeper procedure. Those two facts, one practical and one regulatory, have shaped a great deal of the country's housing.
Why a rear 3 meter house extension lands where it does
Inside, the first metres convert an unusable galley into a room. Beyond about three metres the added floor is further from any window, so you are buying a darker centre unless the roof is bringing light in. Outside, the permitted development limits treat depth beyond the original rear wall as the key test, with a deeper single storey rear extension handled through a separate procedure in which the council notifies the neighbours before work starts.
The roof decides the light and part of the cost
A flat roof is simplest, cheapest and the easiest to fit rooflights into, and modern single ply and liquid systems have made it far more durable than its reputation. A mono pitch throws water away from the house and gives a higher ceiling at the glazed end. A hipped roof, sloping on three sides, suits a wider extension on a house whose own roof is hipped and reads as part of the original building, at the cost of losing internal height near the edges and needing more structure.
How long does a house extension take? A realistic programme
Design and drawings, then either a certificate or an application with its determination period, then the structural design, then tendering, then a wait for a good builder to be free, which is often the longest single wait. On site, groundworks and foundations, superstructure, roof, making watertight, then the opening into the house, then first fix, plaster, second fix and finishes. The part households underestimate is not the building, it is everything before a spade goes in.
Living through it
The wall between old and new normally stays until late, which keeps the house usable for most of the programme. The disruptive weeks are the groundworks, the day the opening is formed, and the first fix. Plan for the kitchen to be out of action, agree working hours in writing, and remember that construction noise is something councils regulate, so keeping the neighbours informed is worth more than it costs.
Questions people ask about 3 meter rear extension
Is three metres always allowed?
No. The limit depends on the house type, the original rear wall and the location, and deeper single storey rear extensions go through a separate notification procedure.
How long does an extension take to build?
The build is usually a matter of months; the design, consents and waiting for a good builder frequently take longer than the construction itself.
Is a flat roof a bad idea?
Not with a modern system properly detailed and with the falls right. Most flat roof failures are detailing and drainage failures rather than material failures.
When is a hip roof extension worth it?
On a wider extension, or where the main roof is hipped and a gable would look wrong. It costs more in structure and loses height at the edges.