Groundwork and Drainage For New Builds and Extensions Back to Blog

Groundwork and Drainage For New Builds and Extensions

Understanding the steps involved in the groundwork phase of construction including scheduling, scheduling building control inspections and drainage considerations.


The important work done before and after a building is constructed

Before you see your new property or extension take shape with layers of brick rising from the ground, much work is required below and at surface level. Clearing the building area (if required), preparing the ground properly, digging the correct foundations, providing the services such as drainage, and provisions for electrical cabling and maybe gas supply are just some of the tasks undertaken before building starts.

Then, once the structure is completed, more groundworks are required to finish things off such as the laying of driveways, paths and more.

Areas of Groundworks

From the start, groundwork-related activities dominate the building of a new property or extension where some type building on previously ‘open’ land is going to take place.

Ground Investigation

A required element of securing planning permission, a ground investigation is required to check the land is suitable for building on. Aspects such as previous uses and whether there is any ground contamination will be covered.

Planning and Scheduling

It’s at this stage you’ll need to have drawn up an effective timetable of work so each specific groundwork activity can take place without being delayed. For example, if you need to extensively clear the area before work can start then you may have to plan ahead to ensure contractors with the necessary equipment such as excavators or tree felling saws are available at the right time. Also, bear in mind your local authority’s building control officer will need to come and inspect the work at various stages (more details below).

Ground Clearance

The area to be built on may require extensive clearance including the demolition of an existing structure. If heavy machinery is required, then access arrangements have to be considered along with the time the contractors require to clear the ground.

Marking the Build

This is a vital next step and has to be accurate. Planning permission will have been granted based on specific dimensions and positioning of the new structure on the area of land. Distances from boundaries, roads and other properties will have been specified and agreed on and no deviance from these measurements would be allowed. Proper marking of the ground usually using flags and perhaps a chalk spray to show where trenches should be dug are usually used.


It’s important to get the drains right to ensure reliable service over many years. Remedial work once construction is complete can be disruptive.

Position of drains - firstly, it’s important to know where existing drains are and - if a totally new build - where your new property would connect to the main system. The services of an experienced drainage company should be sought to help if you’re not sure where the drains run, and to perhaps advise how best to install drainage to the property if your groundworks contractor (if using one) doesn’t fully offer the service.

Drainage installation - ensuring the drains fall to the right gradient is essential; gradients of 1:40 are fine although shallower falls may be possible if using plastic drains. Other drainage considerations include ensuring pipes are laid in straight lines, sharp bends are avoided, and they are laid to an even gradient and provide inspection chambers at changes of gradient and at bends.

Drain testing - a full and exhaustive test of the drainage using water or air pressure should be performed to ensure they perform properly and the joints don’t leak. Check when your building officer wishes to inspect; they may prefer to be present at the initial drains test or when the building is complete - or maybe both.


It may be that significant earth moving may be required to level off and maybe ‘sculpt’ areas of land in preparation for the build. This specialism may be offered by a ground working contractor or a third party may be required.


Where shallower foundations won’t be acceptable for a build - perhaps because of a high water table, too-soft ground and other circumstances where deeper foundations are required - then piling may be required. Piling is where columns of concrete are driven deeper into the ground at regular intervals to provide foundations based on firmer ground at a greater depth. Not all structures require piling, but it’s very important to assess whether yours might - having to provide beefed up foundations subsequent to the build such as through underpinning will prove both very disruptive and very expensive.


The technique deployed and depth of the foundations will of course depend on the type of ground you’re building on, and would have been assessed by your groundwork contractor and further advised on by the building control officer. A popular technique is where concrete is poured into trenches so avoiding bricklaying below ground.  This stage is a good example of the need for effective scheduling as you’ll require the ready mix concrete lorry at a specific time. Ensuring they’re booked at the right time and can access the site easily is obviously paramount. If they can’t get close enough to the trenches pumps may be required to move the concrete, and these will need to be scheduled.

When are Building Control Inspections Required?

They’re required at various stages of groundworks and would usually be organised by your groundworks contractor if you’re using one. Do check for yourself that inspections have been booked at the right time and have been completed before the next groundwork phase begins. There are various ‘milestones’ when inspections usually take place - but check with your local authority as to what their specifics are. Also, check what their preferred notice period is to come and inspect so as to avoid work being held up pending an inspection. The usual inspection points are:

At the commencement of work

When foundations have been dug

When foundation trenches have been filled with concrete

Before oversite concrete (also known as ‘the slab’) is laid

After the damp proof course has been laid

When the drains have been laid in trenches

At the time the drains are being tested

When all work is completed - a final check usually within five days of completion

Oversite Treatment

Before the concrete floor, or ‘slab’, is laid an oversite treatment is required once the foundations have been completed. This usually consists of a hardcore layer of a suitable aggregate such as stone, concrete or brick crushings that are compacted and layered.


Where the flat floor is created carefully through tamping and likely using a vibrating plate.

Other services

Consider the positioning and depth of trenches to house services such as electrical cabling and gas pipes. Plan ahead and allow time for the respective utility company such as telephone, electricity and gas to come and connect your property up to their services.


Clearly you need the services of skilled professionals to help you complete groundworks that will make for secure foundations and an effective drains and services infrastructure for your extension or new build property. Experts to complement your knowledge should be engaged such as drainage professionals and architects; a groundworks specialist canorganise the entire process if you wish.



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