We’ve backfilled the trenches for the new drains. It took longer than expected because the backfill needed to be free of stones larger than 40mm and lumps of clay larger than 150mm. Our soil was full of both, so there was plenty of sorting to do.

We’re now taking up the block paving and moving the soil and stones that couldn’t go back into the trenches. We also need somewhere to store the paving blocks, as we may reuse them elsewhere in the garden.

Bringing in a groundworker

We dug the drainage trenches by hand, but we’ve decided to bring in a groundworker for the foundations. With work, the kids, poor weather and dark evenings, digging them myself would take weeks. A machine should get the excavation done in a day or so.

After getting some quotes, we chose a groundworker with an excavator and dumper. Access has been one of the main challenges: there’s only enough space beside the house for a micro digger.

Fortunately, our next-but-one neighbour has agreed to let us use his driveway, and our next-door neighbour will let us remove a fence panel and bring the digger across their garden. That means we can use a bigger machine, speeding up the work and reducing the cost.

Setting out the foundations

There are three trenches to dig: two six metres long and one ten metres long. The planned trenches are 60cm wide and one metre deep, with a deeper section where the new drainage pipe crosses the foundations.

I’ll measure and mark out their positions, and the groundworker will double-check everything before digging.

We’ll keep the excavated soil for levelling other parts of the garden. Having it removed by a grab lorry would have cost around £1,200!

Planning the concrete pour

Our initial estimate is that we’ll need around four cubic metres of concrete for the 30cm-thick strip foundations. We’ve received a quote of about £500 from a company that mixes the concrete on the lorry.

We expect to have an hour to move it from the lorry into the trenches by wheelbarrow. Based on the estimate we’ve been given of 25–30 barrow loads per cubic metre, that could mean moving as many as 120 loads in an hour.

We considered hiring a pump, but that would add roughly another 50% to the concrete cost.

Checking the neighbouring foundations

One concern has been whether the excavation would fall within the requirements of the Party Wall Act. We wanted to understand how our proposed foundations related to our neighbours’ foundations, and whether a formal notice and surveyor involvement would be needed.

Our neighbours spoke to the builders who constructed their extension a few years ago, and we also dug a trial hole beside their wall to check the foundation depth ourselves. From those checks, we believe their foundations are deep enough that our planned excavation won’t trigger those requirements.

The excavation is due to start in about ten days!

What we’ve spent so far

Our approximate costs to date are:

  • Planning permission, building regulations drawings, structural engineer, building control and insurance: £2,200
  • New tools: £200
  • Demolition: £70
  • New drainage system: £400

We estimate that doing the demolition and drainage work ourselves has saved around £3,000–£3,500 in labour costs. That feels like a good start!