Extension foundations carry the weight of your new walls, roof and floors into ground that can bear it without moving. For most single-storey extensions in the West Midlands that means a concrete trench fill foundation roughly 1m deep, but the right answer depends on your soil, your trees and your neighbours' walls. This guide explains the main types, how depth is decided, and what happens on site, so you can read a quote or a structural engineer's drawing with confidence.
TL;DR
- Most domestic extensions use trench fill foundations. Raft and piled foundations are for poor ground or special cases.
- One local authority guide gives a minimum of 750mm deep, or 1000mm in shrinkable clay. Trees can push that far deeper, up to 2.5m in the guide's worst case.
- Building Control inspects the open trench, never the poured concrete, so book the inspection before you pour.
- Digging close to a neighbour's building may need a Party Wall notice at least one month before work starts.
Table of Contents
- What foundations actually do
- The main types of extension foundation
- How deep should extension foundations be?
- Clay, trees and why the West Midlands needs care
- What Building Control looks for
- Drains, sewers and the neighbour's wall
- Where the money goes
- Our view: foundations are decided before the digger arrives
- Planning an extension in the West Midlands?
- A short checklist before you dig
- FAQ
- Sources
What foundations actually do
A foundation has one job: to pass the load of the building safely into the ground. The Planning Portal puts it plainly: all buildings need adequate foundations, normally concrete, and the design varies from one project to another.
Two questions decide the design:
- How strong is the ground? This is its load-bearing capacity: how much weight each square metre of soil can carry.
- How much will the ground move? Clay swells when wet and shrinks when dry. Tree roots dry it out further. A foundation that sits in soil that moves will move with it.
The second question is the one homeowners tend to miss. A trench can reach firm ground and still fail if it sits in the layer that shrinks every summer.
The main types of extension foundation
Strip and trench fill
A strip foundation is a continuous band of concrete under the walls, wider than the wall it carries so the load spreads out. Traditional strip foundations are dug shallow and then built up with brick or block below ground. Trench fill is the same idea, but the trench is filled with concrete almost to ground level, so there is far less brickwork below ground and the job is quicker.
Trench fill is the workhorse. Local authority guidance notes that for domestic extensions trench fill foundations are the most common. That is why the search "trench fill vs strip foundations" comes up so often: on a typical rear extension the difference is mostly about how much concrete versus how much below-ground masonry you use, and trench fill usually wins on speed and simplicity. Strip with brick footings can still make sense where the trench must be very deep and a full concrete fill would use a great deal of material, or where access for a concrete lorry is poor. Your designer should say which applies.
Pad foundations
Pads are individual blocks of concrete under point loads, such as the posts at the ends of a steel beam. In a modern kitchen-diner with wide bi-fold doors, you will often see pads under the steel supports, with strip or trench fill elsewhere.
Raft foundations
A raft is a reinforced concrete slab that spreads the load across the whole footprint of the extension. It suits weak or variable ground where strips would settle unevenly, and it is sometimes used where deep excavation is awkward. It uses a lot of concrete and steel, so it is rarely the cheapest choice when ordinary strips will do.
Piled foundations
Piles carry loads down through poor upper layers to stronger ground below. They are for the difficult jobs: deep made ground, very soft soils, or heavy tree influence where a trench would have to be impractically deep. A structural engineer should specify them, not the builder on a hunch.
| Type | Typical use | Strengths | Watch for |
|---|---|---|---|
| Trench fill | Most single-storey and many two-storey extensions | Quick, simple, widely accepted by Building Control | Depth in clay or near trees |
| Strip with brick footings | Deep trenches, awkward access | Uses less concrete in deep trenches | More labour, slower |
| Pad | Point loads under steels | Efficient for concentrated loads | Needs ground beams or strips between |
| Raft | Weak or variable ground | Spreads load evenly | Cost and ground levels |
| Piled | Poor ground, major tree influence | Reaches stable strata | Needs engineer design |
How deep should extension foundations be?
There is no single legal depth. The depth is whatever keeps the building stable in your ground, and Building Control approves it on site.
A useful benchmark comes from a council-run building control guide for small single-storey extensions: at least 750mm deep, or 1000mm in shrinkable clay soil, and 600mm wide. The same guide warns that in areas with significant tree influence, depths can be required to go as deep as 2.5m, which needs site-specific structural design.
In practice:
- Firm, non-shrinkable ground, no trees nearby: the shallower end of that range may be accepted, subject to the inspector's view.
- Clay: plan for around 1m as a starting point, and expect more if trees or hedges are close.
- Two-storey extensions: the walls carry more weight, so the engineer checks the width and depth of the foundation against the extra load. Do not assume the single-storey depth applies.
The figures above are a guide for simple extensions. Your engineer's calculation, not a rule of thumb, sets your actual depth.
Clay, trees and why the West Midlands needs care
Much of the West Midlands sits on clay, and clay is the soil that punishes shallow foundations. The NHBC's technical standard explains the mechanism: shrinkable soils combined with trees, hedgerows or shrubs are a hazard to structures that needs special consideration, because the plants pull moisture out of the ground and the clay shrinks.
Three practical consequences:
- Species and distance matter. Tall, thirsty trees such as oaks and poplars have a wide zone of influence. A neighbour's tree counts just as much as yours.
- Removing a tree is not a simple fix. Clay that has dried out can swell again once the tree is gone, which is called heave. Your engineer should advise before anyone picks up a chainsaw.
- Roots found in the trench change the plan. If significant root material turns up during excavation, the depth often has to increase beyond the original drawing, because those roots show active moisture extraction. Ask your builder how a change like this is priced before you sign.
We have written a more detailed look at clay soil foundations for West Midlands homeowners, including the active zone where seasonal movement happens.
What Building Control looks for
Foundations are a priority inspection for a good reason: once they are buried, nobody can see them. A council's building control booking page notes that foundations are among the inspections usually prioritised, and that a site inspection must be requested at least 24 hours before work starts on that stage.
The inspector looks at the open trench, not just the depth. Typical checks include:
- Depth and width against the approved drawings
- Whether the bottom of the trench is the type of soil the design assumed
- Roots, old drains or services crossing the trench
- Proximity of trees and neighbouring structures
The point that catches people out: the inspector will not inspect concrete after it is poured. If you pour before the inspection, you can be asked to dig trial pits or expose the foundation at your own expense to prove compliance. Never let a trench sit open overnight in rain without telling the inspector, and never let anyone pour "to save a day". If you want to understand how site risk and competence are managed on a build more broadly, this guide to construction health and safety training is a useful read.
Drains, sewers and the neighbour's wall
Building over drains and sewers
Old drains turn up in trenches more often than you would expect, and public sewers close to the house can restrict where you build at all. Check before you design, not after you dig. Our guide to building over a sewer and the 3m check explains what to look for.
The Party Wall etc. Act 1996
If your trench is close to a neighbouring building, you may need to notify the neighbour. Section 6 of the Act applies when a building owner proposes to excavate within three metres of a neighbouring structure and deeper than its foundations, or within six metres where the excavation falls inside a 45-degree line from the neighbour's foundations. Notice must be served at least one month before the work starts.
The government's own guidance describes the Act as a way to prevent and resolve disputes in relation to party walls. Skipping notice can halt your build and leave you liable. If you live in a semi-detached or terraced house, assume it applies until a surveyor says otherwise. We cover the timetable in how to avoid party wall delays.
Planning is separate from foundations
Foundations are a building regulations matter, but the size of the building above them is a planning matter. In Birmingham, the council's 45 degree code says that new building work should not cross the 45 degree line, otherwise it would affect a neighbour's outlook and daylight. Run that check before the engineer is instructed, using our six-step 45 degree check, so you do not pay for foundation design on an extension that then needs to shrink.
Where the money goes
Foundations are one of the most expensive parts of an extension, and the least visible. We do not publish fixed prices because the cost swings with ground conditions: a straightforward trench in stable soil and a deep trench with roots and a raft slab are different jobs. What drives the figure:
- Trench depth and width, which set the volume of concrete and the amount of soil to take away
- Whether the excavation hits clay, roots, old drains or made ground
- Access for machines and concrete lorries
- Engineer design fees for anything beyond the standard
- Party wall surveyor fees, if notice is needed
A sensible quote states what depth is assumed and what happens to the price if the trench has to go deeper. That clause matters more than the headline number.
Our view: foundations are decided before the digger arrives
We design and build extensions across the West Midlands, and our view is simple: the foundation is the part of the project where surprises are most expensive, so the work to avoid surprises should happen first.
That means checking the ground, trees, drains and neighbouring structures at the design stage, and agreeing a realistic budget before any site survey turns into a costly argument. It also means one team carrying the job from drawings to handover, so the person who knows why a trench is 1.2m deep is the person on site when the inspector arrives. A divided arrangement, with a designer, a builder and an engineer each pointing at the other, is where unplanned depth increases and missed inspections tend to happen.
If you take one thing from this article, take this: ask every builder who quotes you what depth they have assumed, why, and what it costs per extra 100mm. A good answer shows they have looked at your ground. A vague one shows they have not.
— Raja
Planning an extension in the West Midlands?
We manage design, planning, building regulations and construction under one team, from single-storey and kitchen extensions to double-storey, side-return and wrap-around projects. If you want foundations dealt with as part of a managed process rather than a nasty surprise mid-build, talk to Extension Kings. If you are weighing up an over-garage extension, our extension costs guide for the West Midlands is the place to start. Our guide to extension planning drawings also explains what to budget for before building starts.
A short checklist before you dig
- Confirm planning route and run the 45 degree check.
- Check for public sewers and drains within 3m of the proposed walls.
- Note every tree and hedge within reach, yours and your neighbours'.
- Ask whether a Party Wall notice is needed, and serve it at least a month ahead.
- Get the engineer's foundation design and the assumed depth in writing.
- Notify Building Control and book the foundation inspection before any concrete is ordered.
- Agree in advance how extra depth is priced.
FAQ
What is the difference between trench fill and strip foundations?
Both are continuous bands of concrete under the walls. A strip foundation is poured shallow and the wall is built up from it in brick or block below ground, while trench fill is poured to near ground level so there is little masonry underground. Trench fill is the more common choice for domestic extensions because it is quicker.
How deep do foundations need to be for a two-storey extension?
There is no single figure, because the depth depends on soil, trees and load. Council guidance for small single-storey extensions starts at 750mm, or 1000mm in shrinkable clay, and a two-storey extension should be designed by a structural engineer for the heavier load. Building Control approves the final depth on site.
Do I need Building Control approval for extension foundations?
Yes. Foundations are covered by building regulations, and the inspector needs to see the open trench before concrete goes in. Inspectors do not inspect poured concrete, so booking too late can mean exposing it again at your cost.
Can tree roots change the foundation design?
Yes. In shrinkable clay, roots dry out the soil and cause movement, so foundations near trees often have to be deeper. If significant roots are found when the trench is opened, the depth may need to increase beyond the original plan.
Sources
- Domestic Extensions: a guide to key things to consider, Torfaen Building Control
- Easy Guide for Small Single Storey Domestic Extensions, CNC Building Control
- Building Regulations: Foundations, Planning Portal
- 4.2 Building near trees, NHBC Standards
- Book a building control inspection, Merton Council
- Party Wall etc. Act 1996, section 6, legislation.gov.uk
- Preventing and resolving disputes in relation to party walls, GOV.UK
- 45 Degree Code for Residential Extensions, Birmingham City Council