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Understanding building foundations - where to start
Updated 08/26
By: Content Team
Reviewed: Kim Staples
Every structure needs a foundation matched to two things: the ground beneath it and the weight it needs to carry. Get either wrong, and no amount of good building work above ground will fix it.
What actually determines the right foundation type
The building regulations that apply to foundations
The main foundation types and their trade-offs
A modern alternative worth knowing about
Building over or near drains
Roughly what foundations cost
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Where to start with building foundations
Two factors drive the decision:
Your soil conditions
Your building's size and weight
Soil types include clay, rock, gravel, sand, chalk, and peat, each affecting how deep and what type of foundation you need.
Rock, for example, holds heavy loads well; clay is softer and generally needs deeper foundations.
A soil survey helps confirm this precisely, though it isn't always essential for smaller projects.
Your building's load matters just as much. Heavier structures typically need deep strip foundations, sometimes reinforced with steel, or another suitably strong type.
Foundation building regulations
A few specific factors building regulations require you to account for:
Trees: Nearby trees with deep roots can absorb moisture from the soil beneath your home, causing it to shrink and destabilise your foundations
Soil: Your foundations need to be dug to a depth and type that properly bears your structure's load for the specific soil present
Drains: Foundations should generally be dug at least as deep as any nearby drains or sewers
Other structures: Avoid disturbing neighbouring foundations when digging; matching the depth of nearby structures' foundations is a common way to avoid this
Types of foundation
Strip foundations
The most common type, suited to most soil types and effective for supporting homes and other structures.
Variants include deep strip, concrete strip, wide strip, and reinforced concrete strip.
Pros and cons: Affordable and relatively simple, though installation can take a while and some designs need further support.
Trench fill foundations
A trench filled with concrete rather than built up with bricks, often narrower and quicker to install than strip foundations.
They're commonly used in looser soil or near trees.
Pros and cons: Generally safer than strip foundations, but require a significant amount of concrete.
Raft foundations
Concrete spread across the entire area under a building, rather than confined to a trench.
They're best suited to weaker soil that can't bear a structure's load on its own, common under industrial buildings, and often suitable near trees too.
Pros and cons: Excellent for heavy buildings on unstable soil, sometimes avoiding the need for a separate floor structure, but can be prone to cracking if not installed correctly, and cost can run high.
Pad foundations
Concrete "pads" supporting columns or smaller structures at localised points, well suited to timber frame buildings.
Pros and cons: Quicker to excavate and flexible in shape and size, but can be vulnerable to certain forces, and need to be considerably larger under a heavier load.
Screw pile (helical pile) foundations
A newer option worth knowing about, particularly for garden rooms, smaller extensions, and decking.
Steel piles are screwed directly into the ground rather than requiring excavation and curing time for concrete.
Pros and cons: Much faster to install and less disruptive, with no waiting for concrete to cure, but generally better suited to lighter structures than a full house.
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Foundations for single-storey buildings
Lighter structures like bungalows generally need less foundation strength than taller buildings.
Reinforced concrete strip foundations or reinforced pad foundations are typically the best fit.
Can you build foundations over drains?
This is possible, but the drain's depth and location need checking carefully first.
As a rule, if a drain sits particularly deep, your foundations should be dug to at least an equal depth to avoid destabilising either.
How much do foundations cost to install?
Cost depends on foundation type, depth, and soil conditions. See our new home foundation cost guide for current pricing.
Average new home foundation costs for 2026
New home foundation costs can vary according to a range of factors, including the size of the building, the soil type, the kind of foundations you are laying and accessibility to the site itself.
Before you start
Planning a new construction project?
Getting foundations right is worth the time and cost, since it's the one part of the build you really don't want to redo later.
Checkatrade can help you connect with checked builders experienced in the right foundation type for your project.
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FAQs
Strip foundations, since they suit most soil types and work well for typical domestic properties.
When the soil isn't strong enough to bear your structure's load on its own, or when you're building near trees that could otherwise destabilise deeper foundations.
Generally better suited to lighter structures like garden rooms, extensions, or decking, rather than a full house, though it's worth asking a structural engineer for your specific project.
Yes, provided you check the drain's depth and location first and dig your foundations to at least the same depth to avoid destabilising it.
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