If you are planning an extension, a new build or even a large driveway, the first trade on site will be the groundworkers. Groundworks are the least visible part of a build once it is finished, but they decide whether everything above stays level, dry and crack-free for decades. Here is what they involve, in the order they happen.

What do groundworks include?

Groundworks cover everything below and around a building, up to the point where the walls and floor begin. On a typical extension that means:

  • Site clearance and setting out: removing vegetation, old structures and topsoil, then marking out exactly where the building goes.
  • Excavation: digging the foundation trenches and any reduced-level areas, and removing the spoil.
  • Foundations: pouring the concrete that carries the weight of the building into firm ground.
  • Below-ground drainage: foul and surface water pipes, inspection chambers and connections to the existing system.
  • Service trenches and ducts: routes for water, electricity, gas and data.
  • Substructure walls: the blockwork from the foundation up to the damp-proof course (DPC).
  • The ground-floor slab: hardcore, blinding, damp-proof membrane, insulation and concrete.
  • External works: driveways, paths, retaining walls and the groundwork for landscaping.
Cross-section of typical groundworks for an extension A foundation in a trench carrying blockwork up to the damp-proof course, a layered ground-floor slab beside it, and a drainage pipe bedded in gravel in its own trench. Ground level Concrete foundation in firm ground Blockwall Damp-proof course (DPC) Concrete floor slab Insulation Damp-proof membrane and sand blinding Compacted hardcore Drain bedded in gravel
A simplified cross-section of the groundworks under a typical single-storey extension. Exact layers and depths are set by your design and building control.

In what order are groundworks done?

Every site is different, but on most extensions and new builds the sequence looks like this:

  1. Survey and setting outWe check levels, existing drains and services, and mark the building's position from the approved drawings.
  2. Site clearanceTopsoil, vegetation and any old paving or structures are removed, and the site is made safe and accessible.
  3. ExcavationFoundation trenches are dug to the depth agreed for the ground conditions. Spoil is taken away or reused on site.
  4. Inspection and concreteBuilding control usually inspects the open trenches before the concrete is poured, so the foundations are confirmed to suit the ground.
  5. Drainage and servicesNew drains are laid to the right falls and connected, and ducts are put in for services before the floor goes down.
  6. Walls to DPC and the floor slabBlockwork is built up to damp-proof course, then the floor is built up in layers and the slab is poured.
  7. Handover to the buildThe structure starts on a level, dry, accurate base. External works such as driveways and paths often follow at the end of the project.

How long do groundworks take?

There is no honest single answer, because the timescale depends far more on the site than on the size of the building. The things that make the biggest difference are:

Usually quicker

  • Good access for a digger and wagons
  • Firm, predictable ground
  • Drains that do not need moving
  • A flat, open site

Usually slower

  • Hand-dig only or restricted access
  • Clay near trees, needing deeper foundations
  • Drains to divert or connect at depth
  • Sloping ground needing retaining walls

A good contractor will give you a programme after seeing the site, and tell you which of these factors apply to yours.

What affects the cost of groundworks?

Groundworks are where most unexpected costs on a build come from, because nobody can see underground until the digging starts. The main cost drivers are:

Cost driverWhy it matters
Foundation depthDeeper foundations mean more digging, more spoil to remove and more concrete. Clay soils and nearby trees are the usual reasons for extra depth.
AccessNarrow side passages or rear gardens with no vehicle access mean smaller machines or hand digging, which takes longer.
Spoil removalEvery cubic metre dug out has to go somewhere. Muck away is often a bigger share of the cost than people expect.
DrainageMoving or connecting drains, especially deep ones, adds time and materials.
Sloping groundLevels may need cutting, filling or retaining walls before the building can start.
Unexpected findsOld foundations, buried rubble or soft spots can change the plan once the ground is open.
Compare like with likeWhen you compare quotes, check what each one assumes about foundation depth and spoil removal. A cheaper quote that assumes shallow foundations can end up the more expensive job.

Do I need a separate groundworks contractor?

Not necessarily. Some builders subcontract groundworks to a specialist firm; others, like us, do them in-house. What matters is that the people digging your foundations and laying your drains are experienced in groundworks and are accountable for how the building above performs.

Using one company for the groundworks and the build means nobody can blame "the other contractor" if something below ground affects the structure later, and it removes a handover point where programmes often slip.

From our own work: our drainage works regularly go deeper than 5m, and we build basements on confined sites using temporary earthwork support. The same team that digs the ground builds what goes on it.

Building in GloucestershireMany sites around Gloucester and Cheltenham sit on clay, which shrinks and swells with moisture. On clay, and especially near trees, expect your foundations to be designed deeper than on firmer ground. Read how deep extension foundations need to be.