Single-Storey Extension: Your Structural Engineering Guide

A single-storey extension is the most common residential structural project in the UK. Most of the time it looks straightforward: dig a trench, build walls, span a roof. But the structural engineering behind an extension touches almost every element — foundations, new wall openings, the junction between old and new, the roof structure, and the drainage clearances that often dictate more than people expect.

This guide covers the structural engineering scope of a typical single-storey extension: what gets designed, what gets checked, and what the common complications look like before they become expensive surprises on site.

Why every extension needs structural calculations

Building Regulations Part A (Structure) requires that new structural work is designed to support the loads it will carry without failure, excessive movement, or damage to adjacent structure. For a single-storey extension, this means:

– The foundations must be designed for the ground and the building loads above
– Any new openings — doors, windows, bifolds, roof lights — need beams or lintels sized to carry the structure above
– The existing structure must be assessed where new loads are being applied to it
– The roof structure must be designed to span the extension and resist wind and snow

These aren’t optional assessments. A BCO will check that structural calculations exist and cover the work. Without them, Building Control approval won’t be granted.

Structural calculations also serve the contractor. A good set of drawings and notes means your builder doesn’t have to guess beam sizes, joist spacings, or padstone dimensions. Every site decision that requires a structural judgement is already resolved before they break ground.

Beam sizing over openings: the foundation of most extension projects

The majority of single-storey extensions involve at least one large opening in the existing house — a set of bifold doors, a wide sliding door, or the removal of part of the existing rear wall to open the house into the new extension. Each of these creates a structural opening that needs a beam.

The beam carries the load from the structure above the opening: wall above, floors above, roof above. The load depends on:
– How much floor or roof area the wall carries at this point
– The span of those floors or roof elements
– Whether the opening is in an external wall (usually carrying less load from above) or an internal wall (which may be carrying multiple floors)

For a typical 3m-wide set of bifolds in an external rear wall, the beam may be a relatively modest universal beam — perhaps 203×102 UB or similar. For a 5–6m opening into a kitchen-dining space, the beam gets significantly heavier and longer, and the support points at each end (padstones, masonry piers) need to be designed for higher loads.

The beam specification goes on the structural drawing: section designation, grade, bearing length at each end, padstone specification. The builder orders to that spec and installs to the drawing.

Foundation design for extensions: choosing the right type

The new extension walls need foundations. The appropriate type and depth depends on the ground and the wall loads:

Strip foundations are the default for load-bearing masonry walls. A trench is dug, concrete is poured to a specified minimum width and depth. Width is driven by the bearing capacity of the ground (wider spreads the load on weaker ground). Depth is driven by the need to get below frost, seasonal movement, and root influence zones.

Pad foundations replace strip footings where loads are concentrated at columns or posts — common in extensions with a steel frame rather than masonry walls throughout.

Raft foundations are used on very poor ground or where underfloor heating makes a solid ground floor desirable. A reinforced concrete slab spans across the full footprint, acting as both floor and foundation.

For a typical masonry single-storey extension on reasonable ground (stiff clay, medium-dense sand, or gravel), a strip foundation at 900–1,000mm depth is standard for clay ground and 450–600mm for sandy or granular ground. In areas with mature trees in clay, depth requirements increase significantly — potentially to 1,500mm or more.

Assessing the existing structure where new loads apply

An extension attaches to the existing house. New loads — from the extension’s own structure and from the connection between old and new roofs — land on existing elements: existing walls, existing foundations, existing roof structure.

The engineer needs to check that the existing structure can carry these additional loads. Specific things to assess:

Existing wall at the rear opening: If an existing opening is being widened, or a new opening is being created, the masonry on each side of the proposed opening needs to be wide enough and adequately founded to carry the beam reaction at its end. Thin masonry piers (less than 100mm) can’t carry significant beam loads. Where pier widths are marginal, the structural solution may involve a column within the pier.

Existing foundations at beam bearing points: A new beam transferring load to an existing foundation point needs that foundation to be adequate. The BCO will inspect the exposed foundation during construction and may require the engineer to confirm adequacy once visible.

Existing roof junction: Where the extension roof abuts the existing house wall, the junction detail matters structurally. The extension roof must be tied to the existing wall. The existing wall may need to receive loads from the extension roof on its top — and if it wasn’t designed for that, the engineer needs to check.

Drainage and foundation clearance

Foul and surface water drains are frequently where extensions run into complication. Drains run below gardens from the house to the sewer, and they often run exactly where an extension is proposed. Building Regulations (part H) require that foundations are kept a minimum distance clear of drain lines — both horizontally and vertically — to prevent the foundation loading the drain and vice versa.

The minimum clearance requirements depend on the depth of the drain relative to the foundation and the size of the pipe, but as a starting point: if a foundation is within 1m of a drain, or if the drain is deeper than the foundation, the BCO will almost certainly ask for a structural check of the foundation-drain interaction.

Where an extension absolutely must run over or close to a drain, options include:
– Bridging the drain with a lintel or reinforced concrete bridge below the foundation
– Re-routing the drain before construction starts (this needs approval from the relevant water authority for adoption)
– Replacing the drain with a structurally stronger pipe that can resist foundation loads

Your structural engineer will identify this issue when they review the site conditions. The resolution is usually included in the foundation drawings.

Party walls and structural implications

If your extension is within 3m of a neighbouring property, or if it attaches to a party wall (as in a terraced or semi-detached house), the Party Wall Act 1996 is triggered. You need to serve notice on your neighbour before work starts.

Structurally, extensions that abut party walls often involve foundations that are close to — or deeper than — neighbouring foundations. The engineer may need to design for special foundation details (stepped foundations, underpinning methodology) to avoid undermining the neighbour’s structure.

Party wall surveyors handle the legal and documentation side. The structural engineer provides the technical analysis: foundation details, method statements for excavation adjacent to neighbouring foundations, and confirmation that the proposed work doesn’t adversely affect the neighbour’s structure. These are often requested by the party wall surveyor as part of the award.

Extension roof structure: flat, pitched, and lean-to

Most single-storey extensions use one of three roof forms:

Flat roof (warm deck): Modern flat roofs are built as warm deck constructions — insulation above the structural deck, below the waterproofing. The structural elements are timber flat roof joists spanning between the extension walls and the existing wall of the house. Joist spacing is typically 400mm centres; depth depends on span. The maximum practical span for domestic flat roof timbers without an intermediate support is around 4.5–5m.

Pitched lean-to: Rafters span from a ridge or wall plate at the high end (fixed to the existing house wall) to the eaves at the low end. A lean-to is relatively simple structurally, but the fixing of the ridge to the existing wall needs to be detailed: the rafters push outward at the eaves and downward at the ridge, and those forces go back into the fixing.

Pitched dual roof (forming a valley with the existing): More complex. Valley beams pick up loads from both roof slopes at the valley line. These valley beams need support at their ends and may require sizing calculations in their own right.

Roof lights and glazed roof sections change the loading distribution and may require trimmer framings. Opening-type glazed roofs (with moving gear) add mechanical loads that need to be considered.

What Approved Document A requires for extensions

Part A of the Building Regulations sets out the structural requirements. For a single-storey extension, compliance is demonstrated by:

1. Structural calculations from a qualified engineer, showing the design satisfies the requirements
2. Foundation drawings showing location, width, depth, and concrete specification
3. Structural drawings showing beam designations, joist spacings, bearing details, and any special requirements

The BCO inspects at key stages: excavation (before concrete is poured), steelwork installation (before it’s concealed), and final completion. If the BCO has queries at any stage, those go to the structural engineer — not to the homeowner or builder to answer without professional input.

At PorthouseDean, handling BCO queries is part of every project. Your engineer answers them directly and stays contactable throughout the build. One free revision is included — if Building Control asks for an amendment to the design, that’s covered.

The full engineer scope in summary

For a typical single-storey residential extension, here’s what a structural engineer produces:

– Foundation calculations and drawing — foundation type, width, depth, concrete specification, any drainage clearance details
– Opening calculations — beam sizing for all new door and window openings, padstone specifications
– Existing structure check — confirmation that existing foundations and walls at beam bearing points are adequate
– Roof structural design — joist or rafter sizing, connections to existing walls, any valley or trimmer beam specifications
– Specification notes — timber strength class, steel grade, concrete specs, fire protection, cavity insulation requirements for the structural elements

That complete package is what goes to Building Control and what your contractor uses on site. At PorthouseDean this is delivered in 10 working days standard, with contractor drawings your builder can use immediately — no follow-up calls, no ambiguity about what was meant.