Minimum Foundation Depths for UK Buildings Regulations

Getting the foundations right is imperative for any building construction. It forms the basis for a sturdy and long-lasting build and offers the required stability whilst carrying the weight of the whole construction from the structure to the ground.

But how to decide the nature and specifications of a building foundation before we begin actual construction? Is it that deeper the foundation, stronger the building will be? Another pertinent question is whether the specifications of house foundations need compliance with building regulations. Let’s explore minimum depth of foundations building regulations UK!

What are the Factors that Influence the Depth of Building Foundations?

There are indeed a few tangible and intangible components that play a key role in building house foundations. The type of soil, impediments present inside the soil (large boulders), condition of the land, environmental factors, seismic characteristic, the type of structure, load of the structure, and not to forget the depth of the foundation itself.

The above factors though are all critical for a stable foundation, what’s interesting to note is the depth of the foundation acts as the lynchpin for most of the factors.

How Deep Should House Foundations Be?

As the depth of a foundation is not an absolute metric, but a variable one, there is no silver bullet answer to this. This is because, the depth primarily depends on the type of building, nature & type of soil with the ability to bear the load, and the load of the structure to be shouldered by the foundation. By assessing these factors, one can arrive at an appropriate judgement regarding the depth.

A Shallow Foundation: Shallow foundations do not fully consume the soil and hence remain unstable and become susceptible to jerky movements. As a result, irrespective of the load on the structure, the foundation will not withstand and may eventually lead to subsidence. In addition, frost formation in colder regions, and moisture content in the soil can all easily damage shallow foundations.

A Highly Deep Foundation: On the extreme, if you keep the foundation too deep, the footing may be vulnerable to damage caused by rocks beneath or may also corrode due to extensive contact with water. Another challenge with a highly deep foundation is in cases where the structure requires any maintenance or a repair that warrants accessibility to foundation.

In essence, the foundation of a building cannot be too shallow nor too deep as it may lead to reduced load bearing strength and capacity, thus jeopardising the structural integrity in the long run.

Detailing into Building Regulations

This is exactly where the building regulations come into picture. Though the building regulations offer details on the technical specifications on how a construction project should materialise, they are not really technical guidelines that ascertain minimum depth of foundations building regulations UK.

Building regulations are a set of building standards that act as a guide for compliance, rather than a set technical document. Acting as legislations, they mandate the construction of buildings be made in a way that helps in realising the overall safety, security and longevity of the construction project.

How Building Regulations Act?

The National Housing Board Council (NHBC) for decades has been the official authority helping builders achieve compliance with building regulations to ensure safe execution of any residential or commercial construction project. They act as checks for any inappropriate construction means that may hamper or compromise the structural integrity of the building.

For example, before a construction project starts, an architect or structural engineer should submit the plan to the building control officer with the foundation’s details. The foundation plan will be deemed safe for construction upon thorough inspection by the building control officer. You must know that there are no defined mathematical standards and rules with building regulations. Once the plan is submitted for consultation, the officer is then bound to respond. He/she may either give the go ahead or suggest changes otherwise keeping in mind the minimum depth of foundations building regulations UK.

Few factors that influence are as follows –

1. Soil Type

The type of soil is critical to the foundation depth as it is greatly affected by two factors. One is the maximum load capacity of the structure, and the other is of the weather conditions it is exposed to.

Building on Clay Soil: If your foundation is dug on clay soil, then you must know that clay soil expands and contracts seasonally according to change in weather. Low plastic clays usually expand up to 700mm whereas clay soil with medium plasticity expands up to 900mm. So, for the foundation to be stable, it should be dug up to a level where even and stable moisture content is found. An ideal foundation depth in this case would be 1,000mm.

Building on Sandy Soil: Sandy soil poses a lesser risk as they drain out the moisture content much faster. An ideal foundation depth when dug on sandy soil would be 800mm to 1000mm.

2. Environmental Factors

Frosts: In colder regions frosts can penetrate up to the level of 450mm. As frosts can impact the groundwater and destabilise the structure, it is hence critical to prevent their penetration. It is therefore desirable to achieve a minimum foundational depth of 450mm to realise a safe build.

Trees: Foundations built without the consideration of trees or shrubs will have adverse impacts and thereby damage the building’s structural integrity. If there is close proximity of trees, then foundations will be required to be at least 2m deep. In one of the cases, though the foundations were dug for 1m, the building control advised to dig for a further 2m as there was an oak tree at a 30m distance. So, it really depends.

3. Types of Foundations

Trench Foundations: If the structure to be built warrants a higher load or if the soil condition is not conducive for higher load and requires extra structural support, then trench foundations are suitable. These are entirely concrete filled foundations.

The minimum foundational depth required for trench foundations should be at least 1000mm, and the wall width should have an allowance of 300mm overall (150mm+150mm either side). Wall width + 300mm allowance.

Strip Foundations: Most commonly used for load bearing walls, strip foundations are concrete filled footings characterised by their ability to spread over a larger area. They help in even distribution of structural weight and reduce weight on the soil.

The minimum foundational depth required for strip foundations is 450mm or they should attain the depth until how deep the frost penetrates.

Pad Foundations: Typically used for structures that are light in weight, pad foundations adapt to varied soil conditions. Though being simple foundations, they offer structural stability and strength to areas where the foundations are shallow and are characterised as flat slabs of steel concrete.

The minimum depth required for a pad foundation ranges between 600mm to 2000mm.

Raft Foundations: Raft foundations can be accommodated for all load types as their size depends on the structural load and building dimensions.

The minimum depth of a raft foundation can range from 500mm to 2,500mm. The thickness for raft foundations ranges from 300mm to 1,000mm and depends on the load and nature/type of the soil.

For single-storey extension foundations, a concrete foundational depth of 150mm will suffice. The depth of the foundational trench should be at least 1,000mm.

For two-storey extensions, a concrete foundational depth of at least 200mm is required as it bears more load capacity than a single storey extension. The depth of the foundational trench should be at least 1,000mm.

Getting the foundation right depends on various factors that include, ground condition, type of soil, type of construction, and other environmental factors. There is no definite or concrete answer to this. Building regulations on the other hand are biblical like construction guidelines which guides the safe execution of a building construction taking many factors into consideration. Abiding by the book helps us not only with a long-lasting structure but also to be compliant with governmental standards.

With a plethora of factors affecting, it may be a huge challenge for house owners to determine the foundational depth and relating specifications they need to adhere to. Having handled thousands of new build projects, we at Extension Architecture are aptly placed to assist you on the same.