House Structural Engineers
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Notes Extensions, 4 chapters

Building a rear extension, from idea to first dig

What planning decides, what Building Control decides, what holds the house up once it is opened, and what it all stands on.

House Structural Engineers

2 minute read, then 4 chapters

A rear extension is two projects wearing one coat: what the building is, and how it is built. Different law governs each one, and neither gives you permission for the other.

The chapter map

Four chapters, drawn on the model

Illustrative image: a timber architectural model of a two-storey house in section with a single-storey flat-roofed extension on the right. Two lime lines trace where the loads go: the house roof comes down the outer wall into the new beam at the head of the opening, and the extension roof goes down its own end wall into a foundation and the ground. Four numbered rings mark the chapters: the whole extension outline for planning, the beam for the opening, the foundation under the end wall, and the garden on the extended base board for retaining walls. Each ring is a link. PLANNINGand Building Regulations 1 THE OPENINGthe beam, end on 2 FOUNDATIONSdepth, width, next door 3 RETAINING WALLSin the garden 4
Choose a ring. The house’s load goes into the new beam, and sideways to bearings out of view. The extension’s roof goes down its own end wall, into its own foundation.

Illustrative image

  1. 01Planning and Building RegulationsTwo systems, and what permitted development allows.
  2. 02The opening into the houseThe beam, and where the load goes.
  3. 03Foundations: depth, width, and next doorWhat the base stands on, and how hard it presses.
  4. 04Garden retaining walls: deeper is not strongerWhy width resists the push.

The route, in order

  1. 01Decide what you want. Sizes, light, and where the doors go. Chapter 2
  2. 02Establish the planning route. Only your council can confirm it. Chapter 1
  3. 03Building Regulations, either way. Checked against Schedule 1. Chapter 1
  4. 04The structural design. The opening, the spans, the props, what it stands on. Chapter 2
  5. 05Your neighbours. The Party Wall Act sets notice periods. Chapter 3
  6. 06The dig. Building Control can ask to see trenches before concrete, and a change of garden level needs a retaining wall. Chapter 4

When to call us

While the layout is still a sketch. Once the plan is fixed, the openings and spans are fixed with it, and the structure is fitted around a drawing instead of shaping it. We design to meet the Building Regulations without weight it does not need. Start with this

Send us the sketch before it becomes a drawing.

Fixed fee, confirmed in writing before we start. We reply within one working day.

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