What is an extension?

An extension is an addition to an existing building, where a new room is built directly onto the existing external wall. Common examples include an extra living room, kitchen or bedroom added to the rear or side elevation.

Extension vs. outbuilding

• Extension — an addition directly attached to the existing building, becoming part of the dwelling
• Outbuilding — a separate, freestanding structure on the property, such as a shed or garage

Application

An extension is a popular way to increase living space:

• Rear extension — the most common form; extending at the rear of the house
• Side extension — extending at the side, often on corner plots or detached houses
• Conservatory — a lightweight extension with lots of glass, as a transition between inside and outside

Construction

An extension typically consists of:

1. Foundation — usually a strip or pad foundation, independent of the existing foundation
2. Walls — masonry (cavity wall) or timber frame
3. Roof — flat roof (EPDM or bitumen) or mono-pitch roof with tiles
4. Junction — the joint between old and new must be watertight, often with an expansion joint

Planning permission

Not every extension is permitted development. The rules depend on:

• Floor area of the extension
• Height and depth
• Distance to the boundary
• Local planning regulations

Always check with your local planning authority before starting work.

Related terms

• Planning permission
• Foundation
• Strip foundation
• Timber frame construction
• Flat roof

Learn more about building terms in our knowledge base at fredsdiyplans.com.