Rear Extension Structural Logic: Steel, Timber, or Both?

Every rear extension involves a structural conversation. The moment you decide to open up the back of your house, you are asking the existing structure to behave differently — redistributing loads, spanning openings, and supporting new roof and wall elements. Getting the structural logic right determines not just the safety of the build but also the cost, the construction programme, and the quality of the finished space.

Opening Up the Rear Wall

The most significant structural move in a typical rear extension is removing part or all of the existing rear wall. In a Victorian or Edwardian terrace, this wall is usually load-bearing: it carries the first floor joists, the roof structure above, and sometimes the lateral restraint for the party walls.

To remove it, you need a beam — usually a steel universal beam (commonly called an RSJ, though strictly that is an older profile) — that takes the loads from above and transfers them down through the structure to the foundations. The beam sits at ceiling level, and its size depends on the span, the loads it carries, and the acceptable deflection.

A structural engineer will calculate the beam size, specify the bearing details at each end, and determine whether the existing walls and foundations are adequate to take the concentrated point loads. This is not a job for rules of thumb or builder's intuition. The calculations are specific to your house, your proposed opening, and your loading conditions.

Steel Beams: Sizing and Placement

Steel is the default material for spanning large openings because of its strength-to-depth ratio. A steel beam can span five or six metres with a relatively modest section depth, keeping the finished ceiling height as generous as possible.

The beam is typically supported on padstones — concrete or engineered stone blocks that spread the point load into the masonry below. If the masonry beneath the padstone is in good condition and the foundations are adequate, this is straightforward. But in many older houses in Hampstead, the existing masonry is lime-mortite bonded, the brickwork below window openings may be thinner than elsewhere, and the foundations may be minimal rubble footings only 300–450mm deep.

In these cases, the structural engineer may specify a concrete padstone cast in situ, a steel post or column to transfer the load to a new foundation pad, or localised underpinning beneath the bearing point. Each of these adds cost but is essential for structural integrity.

Timber and Hybrid Structures

Steel is not the only option. Engineered timber — glulam beams, LVL (laminated veneer lumber), or CLT (cross-laminated timber) — can span smaller openings and is used extensively for the roof structure of extensions. Timber has the advantage of being easier to work with on site, lighter to lift into position, and warmer to the touch if left exposed as a design feature.

For the main structural opening into the existing house, steel is almost always the pragmatic choice. But the extension's own structure — its roof beams, rafters, and any internal divisions — can often be timber. A common arrangement is a steel primary beam at the junction between old and new, with a timber flat roof structure spanning from that beam to the new rear wall.

Hybrid structures work well when designed as a coherent system. The structural engineer and architect should collaborate so that the steel and timber elements connect cleanly and the load paths are clear and unambiguous.

Foundations for Extensions

The extension itself needs foundations, and these must be designed for the ground conditions at your site. In north London, heavy clay soils are common, and the National House Building Council (NHBC) guidelines specify minimum foundation depths based on proximity to trees and the plasticity of the clay.

A typical strip foundation for a single-storey rear extension in clay soil will be at least 1 metre deep, and often deeper if there are trees nearby. The foundation width depends on the loads and the bearing capacity of the soil. Your structural engineer will specify these, and building control will inspect the trenches before concrete is poured.

If the new extension foundations are deeper than the existing house foundations — which is common — there is a risk of undermining the existing structure during excavation. The contractor may need to dig in short sections, pour concrete quickly, and use temporary support to prevent movement. In some cases, formal underpinning of the existing foundations is required.

Underpinning: When and Why

Underpinning means strengthening or deepening existing foundations. It is needed when the new construction affects the stability of the existing footings, or when the existing foundations are found to be inadequate during the works.

Traditional mass concrete underpinning involves excavating beneath the existing foundation in short alternate sections (typically one-metre bays), pouring concrete to the new required depth, and allowing each section to cure before moving to the next. It is slow, labour-intensive, and disruptive, but it is well-understood and reliable.

Mini-piled underpinning is an alternative where access or ground conditions make traditional methods impractical. It is more expensive but faster and less disruptive to the existing structure.

Not every extension requires underpinning. A good structural engineer will assess the existing foundations (sometimes by digging a trial pit) and determine whether they are adequate for the new loading conditions. If they are, underpinning can be avoided — saving significant cost and time.

The Structural Engineer's Role

The structural engineer is one of the most important professionals on your project. They calculate beam sizes, specify foundation depths, design connections between new and existing structures, and ensure the whole system works as intended. Their drawings and calculations are submitted to Building Control for approval and are used by the contractor to build accurately.

Choose a structural engineer with experience of residential alterations in your area. Period properties in Hampstead have particular quirks — shallow foundations, inconsistent brickwork, timber lintels that have degraded over time — and an engineer who has worked on similar houses will anticipate these issues rather than being surprised by them.

We help homeowners in the Hampstead area connect with structural engineers and architects who work together effectively. The best projects are those where the architectural ambition and the structural logic are aligned from the start, not resolved through awkward compromises on site.

Key Takeaways

The structural approach for your rear extension depends on the span of the opening, the condition of the existing house, and the ground conditions. Steel beams handle large openings efficiently. Timber works well for the extension's own structure. Foundations must suit the clay soils common in NW3. And a capable structural engineer is not an optional extra — they are fundamental to the project's success.