In short
The forms an extension can take — rear or side extensions, an extra storey, an attic, a basement, or enclosing an existing covered area — and the controls that decide how much you can add: setbacks, site coverage, envelope, site features and special area rules. It covers the structural question planning permission does not answer, how an extension is treated when most of the house is rebuilt, and how to find out where you stand before drawing anything.
The forms an extension can take
- A rear or side extension. The most common, adding ground-floor space for a larger living room or kitchen, and often the change that transforms how a house works. Limited by the rear and side setbacks and by site coverage.
- An additional storey. Possible where the envelope allows another storey and the existing structure can carry it, or be strengthened economically. Both must be true, and the second is answered by investigation, not optimism.
- An attic. Often the best value, because much of the volume already exists. Its form is governed by the roof envelope rather than treated as a free storey.
- A basement. The most expensive space you can add: excavation, retaining structures, waterproofing and monitoring, with significant engineering on a tight plot. Sometimes the only route to the space you need.
- Enclosing a covered area. Balconies, terraces and car porches are often the cheapest extra floor area on the plot, though enclosing them usually counts towards floor area and coverage, so it is a planning matter.
What decides how much you can add
- Setbacks from the road and the side and rear boundaries, which vary with the housing type. On a narrow plot they take a large share of the site.
- Site coverage — the maximum share of the plot the house may occupy.
- The envelope — storeys, height and roof form, which together set the three-dimensional limit.
- The existing structure, and whether its frame and foundations can carry more.
- Site features: drainage reserves, road line reserves, protected trees and sewers, all of which remove developable area.
- Special areas — Good Class Bungalow and conservation areas carry their own rules.
Guidelines are revised from time to time, so confirm what applies to your property with your qualified person rather than relying on a neighbour's extension.
The structural question
Planning permission establishes what you may build. It says nothing about whether your house can carry it. Many older houses stand on foundations sized for two storeys and a light roof.
A new storey, a heavy stone floor or a pool changes those loads. Where capacity falls short, there are remedies — strengthening columns, new foundations threaded between the old, transfer structures — and none of them is cheap or quick. Investigate before you design.
When an extension becomes a reconstruction
An extension is normally treated as A&A work as long as the original house substantially remains. Where so much is rebuilt that the house is effectively new, it is treated as a reconstruction or new development, and current guidelines apply in full — which changes what you may keep. Settle which it is with your qualified person at concept stage.
How to find out where you stand
A feasibility study works from your title and survey plan, the approved plans where they exist, a measured survey where they do not, and the guidelines that apply to your plot. It should tell you the permissible envelope, the floor area you could achieve, the setbacks and site constraints that apply, and whether the existing structure can support what you want.
It is inexpensive relative to the project, and it prevents the most common failure: falling in love with a design the plot cannot hold.
Key takeaways
- Most landed houses can extend; the plot's envelope decides by how much.
- An attic is often the best value, because much of the volume already exists.
- Planning permission says what you may build, not whether the house can carry it.
- Establish the envelope first. Everything drawn before that is a guess.
Questions people ask
Layout reconfiguration, rear extensions, attic conversions, lifts, staircase changes, facade upgrades, kitchen transformations and whole-house interior upgrades.
