Strip Footing Foundation Specifications for Building Regulations

Strip footings are the most common foundation type for domestic extensions in the UK. Getting the width right is not guesswork — it follows a structured process using Approved Document A and your project’s specific wall loading.

The Starting Point: Approved Document A Table 10

Approved Document A contains a simplified table (Table 10) that sets out minimum strip footing widths for different ground types and different levels of wall loading. To use it, you need two things:

1. The type of ground beneath the site (Condition I through IV, from firm rock to soft clay)
2. The total load the footing must carry, expressed as a line load in kN per metre run of wall

The table then gives a minimum foundation width in millimetres. This is the floor, not the ceiling — you can always go wider.

The PorthouseDean Minimum

PorthouseDean’s standard minimum recommendation is 600 mm wide, regardless of what the table requires for lighter loads. This covers most ground conditions encountered on domestic projects and provides a sensible margin above the theoretical minimum. For heavier loads or poorer ground, the required width will be greater.

Four Conditions Must Be Met to Use the Table

Approved Document A’s simplified approach is only valid when:

1. The supported wall is not more than 215 mm thick (i.e., a standard brick or 100 mm blockwork leaf)
2. Floor joists span no more than 4.5 metres from the foundation wall
3. The roof span is no greater than 10 metres
4. The first floor uses timber construction (not concrete)

If any of those conditions is not met — for example, a longer floor span, a wider wall or a concrete first floor — the simplified table cannot be used and the foundation must be sized by calculation.

A Worked Example

Consider a two-storey rear extension on a site with medium-dense sand (Condition II ground) and standard domestic loading:

– Table 10 for Condition II and a typical two-storey wall load gives a minimum width of around 400 mm
– PorthouseDean adopts 600 mm as standard
– The drawings specify: 600 mm wide × minimum 600 mm deep strip footing in C35 concrete

This is the specification that goes on the structural drawings and is submitted to Building Control.

Depth Alongside Width

Width alone does not complete the specification. Depth matters too:

– Minimum 450 mm below ground level on most soils (to avoid frost heave)
– Minimum 900 mm on clay
– Deeper where trees are present — calculated against the species and distance using the LABC foundation depth calculator or similar tool

The structural drawings will state both dimensions, and the BCO will confirm adequacy on site inspection.

Get Your Foundation Specifications Designed

PorthouseDean produces foundation specifications for domestic extensions and new builds across England and Wales. Get a quote to get your structural drawings produced.