How to Check Foundation Work: A Homeowner's Inspection Guide

Foundations are the one part of your building project you'll never see again. Once the concrete's in and the walls go up, everything below ground is buried - for good. Which is exactly why this is the inspection stage you absolutely cannot afford to miss.

The good news: you don't need an engineering degree to spot the most common problems. You need to know what to look for, when to look, and crucially - to look before the concrete truck arrives.

TL;DR Checklist

• Trench depth matches soil type and tree proximity (450mm frost minimum, 900mm-1.5m typical, up to 2.5m on clay near trees)
• No standing water, soft spots, or contamination in the trench
• Building control has inspected and approved BEFORE concrete is poured
• Trench sides are stable and not collapsing
• Concrete mix confirmed as correct (typically C25/30 for strip foundations)
• Concrete depth adequate - minimum 150mm, typically 225mm+
• Drain runs positioned and sleeved before pour
• Steel reinforcement matches structural engineer's drawings (if specified)

The Full Guide

What determines foundation depth?

Three things drive how deep your foundations need to be: soil type, tree proximity, and what you're building on top.

On stable, non-shrinkable ground with no trees nearby, strip foundations at around 600-900mm deep are common. On shrinkable clay - which covers a huge chunk of southern and central England - you're looking at 900mm to 1.5m as a starting point. Add a mature oak or willow within influencing distance, and depths of 2m to 2.5m aren't unusual.

Your structural engineer should have specified the foundation depth based on a site investigation. If nobody's done a site investigation and your builder is "just digging to the usual depth," that should set alarm bells ringing.

Before the concrete arrives

This is your window. It might be a few hours between the trench being dug and the concrete truck turning up. Make sure you're there.

Check the trench bottom. Walk along the length (carefully - don't fall in). The bottom should be firm, level, and clean. If you can push a heel into it easily, the ground is too soft. If there's standing water, it needs pumping out before the pour - water weakens concrete as it cures. If you see old drain runs, rubble, timber, or anything organic, these need removing and replacing with compacted material or the foundation redesigned around them.

Check the trench sides. They should be vertical and stable. If clay or sandy soil is crumbling in from the sides, the effective width of the foundation is being reduced. On deep trenches, timber shoring may be needed for safety and quality.

Look for made ground. "Made ground" is builder's terminology for soil that's been previously disturbed or filled - old garden waste, demolition rubble, or imported fill. Foundations on made ground need special treatment, usually deeper excavation to reach undisturbed soil or an engineered solution like piled foundations.

The critical inspection: building control

This cannot be overstated: building control must inspect the foundation trench before concrete is poured. This is not optional and it's not flexible. Once concrete is in, it cannot be inspected, and it cannot be easily undone.

Your builder is responsible for calling building control at the right time. But here's the thing - if they don't, nobody will chase them. The concrete goes in, the walls go up, and it's only when you try to get your completion certificate that someone notices the foundation stage was never formally signed off.

Tree proximity

Trees near foundations cause more structural problems in the UK than almost anything else. Clay soil shrinks when tree roots extract moisture (subsidence) and swells when trees are removed and moisture returns (heave). Both crack walls and distort structures.

NHBC Chapter 4.2 provides detailed tables linking tree species, mature height, and required foundation depth at various distances. As a rough guide:

• High water demand trees (oaks, willows, poplars, elms): influence extends to 1.25 times mature height
• Medium water demand trees (ash, beech, cherry, lime): influence extends to roughly equal to mature height
• Low water demand trees (birch, holly, elder): influence extends to about 0.5 times mature height

If your site has existing trees within these distances, your foundations should be deeper - often significantly. If trees have been recently removed, heave precautions (compressible material around the foundations) may be needed.

The concrete itself

When the mixer arrives, check the delivery ticket. It should state the concrete mix, which for most domestic strip foundations is C25/30 (sometimes referred to as GEN3 or ST3). The ticket will also show the volume ordered, the slump (a measure of workability), and any admixtures.

The concrete should be poured in a continuous operation - not half today and half tomorrow. It should fill the trench to the specified depth, typically at least 225mm for strip foundations. The top surface should be reasonably level, as this is what the first course of bricks sits on.

Drain runs through foundations

If any drainage needs to pass through or under the foundations, it must be planned and positioned before the pour. Pipes should be sleeved (placed inside a larger pipe) so they can move independently of the foundation - buildings settle slightly, and rigid connections between drains and foundations crack.

Cutting through a foundation after it's been poured to run drainage is a structural compromise that requires engineering sign-off. It's far better to get the drainage position right from the start.

Common Problems

Pouring without building control inspection. The most common and most consequential mistake. Your builder might say "they'll come and see it later" or "I've done hundreds like this." Doesn't matter. No inspection means no formal record, and potentially no completion certificate without costly remedial work.

Insufficient depth near trees. Cutting corners on depth saves a few hundred pounds in excavation costs. The subsidence claim five years later costs tens of thousands and is a nightmare to live through. If the site has trees, the foundations must account for them.

Water left in the trench. A bit of moisture won't harm concrete, but standing water dilutes the mix at the bottom of the trench, creating a weak layer right where you need the most strength. Trenches should be pumped dry before pouring.

Wrong concrete mix. If the delivery ticket says GEN1 and the drawing says C25/30, don't let the pour proceed. The wrong mix might look the same going in but will have different strength characteristics. This is a straightforward check - read the ticket and compare it to the specification.

Foundation work should always be covered in your builder's quote. If your quote doesn't reference building regulations at all, read our guide on quotes that miss building regulations - it's one of the most common and costly oversights.

Questions to Ask Your Builder

1. "Has building control confirmed they can inspect before we pour?" - Establishes whether the inspection is booked without being accusatory.

2. "Can I see the structural engineer's foundation drawing?" - You're entitled to see this. It should be on site. If it's not, that's concerning.

3. "What depth are we going to, and why?" - A good builder will explain the soil conditions and any tree influence. If the answer is "the usual" with no reference to site-specific factors, push for more detail.

4. "Is the ground what the engineer expected?" - Sometimes site conditions differ from what the desk study predicted. If the soil is softer, wetter, or different from expectations, the foundation design may need revising.

5. "Are any drains passing through the foundation?" - If yes, confirm they're sleeved and positioned correctly before the pour.