What does building control look at in a foundation trench?

On my build the inspector came out before any concrete was ordered and walked the whole trench looking at what the ground was actually made of. He flagged tree roots and soft, wet subsoil in one corner, told the builder to scrape the trench bottom back to firm ground, and required extra depth there. Nothing was poured until he had seen it again in photographs.

What happens if building control finds tree roots in your foundations?

You dig deeper in that area until the ground is something the inspector will accept, and you carry the cost. On my build the far corner went deeper and the soakaway had to be taken further down as well, and both were decided on site rather than on any drawing.

What is a build over agreement and when do you need one?

It is permission from the water company to build over or close to a public sewer. Ours ran across our own ground, under the corner of the extension and into a manhole on the driveway, and because it also served next door it counted as public. The application to Thames Water cost £299 and needed drawings showing the sewer, the manhole and the new wall together.

How much did foundations and blockwork up to DPC cost?

The pre-mixed concrete for the foundation pour was invoiced at £2,650 and the blockwork up to damp proof course at £6,450, which I paid in two parts. Those are 2021 prices in Oxfordshire on a 55m2 kitchen extension, so read them as a shape rather than a quote.

Groundwork on a Real Extension: Foundations, Sewers and Slab

The ground is the one part of an extension nobody can specify in advance. Everything else on this build had a drawing behind it. The trench had a survey, an assumption and a price, and within two days of the digger arriving all three had been overtaken by what was actually down there.

Work started on 20 October 2021, a month later than promised. The builder marked the footprint out in yellow line, a one tonne digger came off a trailer, and the perimeter trench went in over the following week.

A deep foundation trench dug around the perimeter of an extension footprint, with exposed pipework against a brick wall, spoil heaped either side and a manhole cover in the foreground. — The perimeter trench. The water pipe running the full length of that side was cut through twice during this excavation.

Somewhere in that week the water main was cut. Then it was cut again. The pipe ran the full length of the side of the extension, which nobody had established before the machine went in, and it was found the way buried services usually are. The builder drove to the merchant for fittings both times, and both times we had to locate our water meter, hidden at the front of the property, and shut the supply off before anything could be repaired.

Warning

A verbal warning that there is a pipe somewhere along that side is not a service protection measure. Mark buried runs on the ground with paint or tape before any machine arrives, and hand the operator a plan with those runs drawn on it.

The inspection that stopped the concrete

The building control officer inspected the foundations on 28 October. The far right corner, which is where the bifold doors eventually went, had tree roots and soft ground in it. That corner was dug deeper, and he said the soakaway needed to go further down as well.

A spade standing upright in the bottom of a deep excavated corner, with tree roots and broken brick visible in the exposed trench walls. — The far right corner, dug deeper after building control raised concerns about roots and ground conditions.

Then the trench filled with water. Photographs went over on 29 October and the instruction that came back was to bottom the trench up: scrape the base back to firm ground and take the soft subsoil out before any concrete went near it. He approved the pour once that was done.

That one visit is the whole value of the building control relationship. Nobody else on site had a commercial reason to stop the concrete, and once concrete is in a trench, whatever it is sitting on becomes permanent.

The sewer under the corner

The same afternoon's photographs caught the thing that cost me most in paperwork.

An excavated trench showing a drain run passing under the line of the new extension wall towards a manhole cover on the driveway. — The sewer passing under the extension corner towards the manhole on the driveway. Because it also served next door, it was public, and that meant a Thames Water build over agreement.

The sewer crossed our own ground, ran under the corner of the extension and into a manhole on the driveway. It also took next door's waste, and that is what made it public rather than private. The application to Thames Water cost £299 and needed drawings showing the sewer, the manhole and the new wall in relation to each other. It was a painful process, made worse by how close the sewer sat to the exterior wall.

What I got wrong was the order. The application went in on 20 July 2022. Work had started on 20 October 2021.

9 months

The gap between breaking ground and applying for the build over agreement. By then the foundations, the blockwork and the roof structure were in, and the prompt to contact Thames Water had come from the building control officer in May 2022 rather than from anyone on my own side of the job.

Check for public sewers before the design is finalised. Mapping is cheap, the agreement is cheap, and the exposure if you build over one without permission is neither.

Out of the ground

Concrete went into the foundations on 2 November, invoiced at £2,650 for the pre-mix. Blockwork started on it the next day.

Below ground blockwork rising from a poured concrete foundation along a side return, with a mini dumper and spoil heaps in the background. — The side return after the pour: first courses of below ground blockwork, holes formed around the crossing gas and electricity pipes, and a pier being built.

Next door's gas and electricity supplies crossed our footprint below ground, so the builder formed openings and built around them. They are still down there, invisible unless somebody digs. The pier partway along the long side return started here too, partway along that long run of wall. On 6 November the digger caught the new blockwork and a couple of courses had to come down and be relaid.

The invoice for blockwork up to damp proof course was £6,450. I paid £4,000 of it on 12 November and the remaining £2,450 on 7 December. Stage payments do not have to leave your account the day the invoice lands.

The lintel I had to ask for

A concrete lintel bridging an opening in low blockwork above a drain run, with courses of brick built over the top and a string line set out along the wall. — The lintel bridging the existing drain run near the manhole, installed after I raised it and building control required it.

Where the new wall crossed the existing drain, it needed a lintel to bridge the pipe so the weight of the wall was not carried by the drain. It was not going to be built. I recognised the detail from watching a family member build years earlier, raised it with the building control officer on his November visit, and building control required the builder to put it in.

Tip

You do not need to know how to build in order to catch something like this. You need to ask the question out loud to the one person on site whose job is to say no. Building control will answer a homeowner's question, and their answer carries weight yours does not.

Oversite, blinding and a slab full of water

The porch came down sometime between 7 and 12 November and its bricks were broken up and used as hardcore in the new floor. On 23 November the building control officer looked at the oversite and said there were too many undulations in it, so levelling and more sand went in on 26 November. The porch pieces too large for hardcore turned out to be unsuitable for a grab lorry as well, and on 1 December they needed a specialist removal run nobody had planned for.

A tape measure held against blockwork inside the extension footprint, showing the distance between a marked blue level line and the compacted hardcore surface below. — Checking the sub-floor level: roughly 23cm between the blue line marking finished blockwork level and the top of the hardcore.

Then the base was blinded with sand, the layer that stops sharp edges in the hardcore working their way through the membrane.

A rectangular area of compacted hardcore covered in a smooth layer of sand, bordered by blockwork, with a plate compactor and tools nearby. — The base blinded with sand. A couple of small stones still sit proud, and those are what pierce the DPM.

The slab was poured on 30 November. Two days later it rained, and I sent the builder a photograph of standing water lying in pools across it. That morning is the picture at the top of this chapter: pooled rain, the ribbing of the levelling tool still showing, the DPC folded up over the first course of inner blockwork with loose blocks holding it flat.

His answer was that the levels were not far out, that getting a floor this size truly flat would need a machine nobody brings to a domestic job, and that the levels get corrected by what goes on top: sand over the slab, rigid insulation, underfloor heating, then liquid screed once the roof is on and the room is sealed. He was right, and the screed did fix it a year later. I still spent weeks convinced I had bought a permanently uneven floor. Trades stopped for Christmas on 15 December 2021 with the shell out of the ground.

Groundwork does not end when the slab does

Nine months later I was still digging. On 2 November 2021 the builder had agreed to dig out the trenches for the gas and electricity service alterations. He never did, and nobody chased it because it sat on no one's programme. In July 2022 I found a groundworks contractor through MyBuilder instead. The quote was signed on 21 July at £3,250: service trenches for the electric duct, the gas main and the water main, a one square metre soakaway wrapped in terram and filled with clean stone, two back inlet gullies into the sewer, and making good afterwards. He was on site from 25 July.

I paid the £3,250 as £3,000 in cash, drawn out over several trips to a cash machine, plus £250 by transfer. There was no invoice and no receipt. The work was good and the price was fair, and I would still not do it that way again.

A narrow trench beside a brick wall containing a new yellow gas service pipe and a black corrugated conduit, with the old gas pipe cut off. — The replacement gas service going in during July 2022, old pipe cut off, trench dug ready for the network's own crew.

On 26 July gas was smelled in the trench. SGN came out as an emergency, the scheduled works already booked were cancelled, and the gas was rerouted while next door's supply was tested. Whether the leak pre-dated the digging was never settled. SGN attributed it to the builder, and I am genuinely unsure. What it did in practice was bring forward work I had already paid for.

Close-up of a shared trench containing gas, electricity and water services alongside an old ceramic sewer pipe repaired with a plastic section and flexible couplings. — Gas, electricity and water sharing one trench, beside the old ceramic sewer cut and repaired with a plastic section. The separation looks tighter than it should, and it passed inspection.

Two more pieces of groundwork happened long after everyone assumes groundwork is finished. Where a steel had to land inside the old kitchen, the existing slab was too thin to carry it, so the builder cut through, lapped the DPM and poured a proper base underneath. The labour and membrane came to £480 in September 2022. The same month the pier foundation was extended, dug 500mm down and 300mm out for £380, and my own notes record that the existing foundation already ran 140mm past the original base, so it may not have been strictly necessary.

Freshly poured concrete filling a rectangular excavation cut through an existing floor slab, with overlapping damp proof membrane and pipework around it. — Concrete poured where the existing slab was cut through, with overlapping DPM, so the steel bears on a thick base rather than a thin floor.

The last of the digging exposed the drainage the house had been living with for decades.

An excavated trench alongside a brick wall exposing old ceramic drainage pipes with several branch connections, partly filled with water. — The existing sewer opened up in October 2022: ceramic pipe, several branches, and the run coming down the back of the house into it.

The ground writes the first month of your programme, not your builder and not your architect. Every surprise here was invisible on the drawings and obvious within an hour of a machine turning up. Budget time and money for a trench that disagrees with the survey, get building control out before any concrete is ordered, and photograph everything before it is buried.

When your own build reaches this point, work through the groundwork phase guides. They cover setting out, foundation depths, drainage and build over agreements, the oversite build-up and the DPC, in the order the work happens on site.

Groundwork

The inspection that stopped the concrete

The sewer under the corner

Out of the ground

The lintel I had to ask for

Oversite, blinding and a slab full of water

Groundwork does not end when the slab does

Frequently asked questions

Do this phase on your own build

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