How to Excavate a Basement

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With so many people looking to extend their homes, we thought it helpful to show how to excavate a basement to create a habitable space. Every basement will be different, but this small conversion illustrates many of the principles involved.

basement excavation

Existing cellar, basement or undercroft

Many properties ripe for conversion have an existing space beneath the house.  This may arise from a difference in ground level between one side of a house and the other. In this case there was an undercroft 1.5m deep at the rear garden end, tapering to a crawl space over a distance of about 6m. It had been partially excavated at the near end to allow us to make better use of the space for storage.

Undercroft before basment conversion

 

Does a basement require planning approval?

There is no single answer to this. Planning approval is unlikely to apply where you are simply excavating without any change to the external appearance of a property, and you are not creating separate accommodation. However, where your excavation does change the external appearance (even for a lightwell), or creates separate accommodation, then you may require planning consent. It is always best to check with your particular planning authority first. In this case, we submitted a pre-app to the council which confirmed that planning approval was not required.

Structural calculations

You should seek professional help for statutory requirements such as building control and any Party Wall matters. A structural engineer will produce calculations and drawings, with a method statement for carrying out the work. Calculations will account for soil conditions and supported loads to determine the extent of underpinning and sizes of steels and padstones. Showing your neighbours that you are engaging professionals should also give them confidence.

In this case we first dug several pilot holes in different locations near the existing walls to investigate the depth of the original foundations. We found the Edwardian foundations to be virtually non-existent. This helped the structural engineer to plan the design and the degree of underpinning required.

Party Wall Act

If you are excavating near a neighbour’s buildings, the Party Wall Act is likely to apply. In essence, there are two key criteria for excavations that bring the Act into play, which we only summarise here:

  • if excavating within 3m of a neighbour’s building or structure where your excavation goes deeper than the neighbour’s foundation.
  • if excavating within 6m of a neighbour’s building or structure where your excavation goes deeper than a line drawn downwards at 45° from the bottom of your neighbour’s foundation towards your excavation. If that line meets your excavation, the Party Wall Act applies.

If you search on-line for “Party Wall Act” you’ll find authoritative guidance; there’s a particularly good explanatory booklet which is usually top of the search. This contains illustrations of the various scenarios and provides template letters which you can use to notify your neighbours formally. It is well to follow this process for your own protection and peace of mind.

Party Wall Act as applied to basement conversions

Hopefully, you can talk through your project with your neighbours and they will be happy to sign the consent letter. If not, you may be faced with paying for Party Wall surveyors. In this case study, one set of neighbours was content but the other wanted to dispute the plans. In the end everything was fine, but we had to pay for two Party Wall surveyors, one for each side of the Party Wall. Apart from the formalities, the surveyors also took photographs of the neighbours’ side of the wall before start of works, in case of any dispute later.

Building Control

Building control will apply to your basement conversion. Initially, they will wish to see the structural drawings and calculations, plans for drainage, fire escape routes, ceiling heights and damp proofing. Again, if you have employed professional help, these matters should be covered in your drawing pack. Once work has started, the building control officer will give you a list of stages that he or she will wish to examine before you progress to the next stage. At the end of the project, building control will sign off the project; a vital certificate that you should append to your house documentation in the event of a future sale.

building contractor

Any reputable building contractor can install a basement. Working to a complete drawing set and technical specification, this is standard fare. For this project, we wrote the technical specification to cover all matters that weren’t included on the drawings; matters such as electrics, decoration, flooring, home network and other requirements. Only with a complete technical specification can a contractor provide a realistic quotation for the work. In this case, we worked to a CIOB Small Works Contract which you can order on-line.  There are others such as the JCT Minor Works Contract.

Basement Excavation

Depending on access, there are several ways to excavate a basement physically. In some cases, it is possible to remove a section of wall temporarily and install a conveyor belt (a bit like you see in a coal mine). Labourers dig the spoil onto the conveyor belt which then dumps it outside, usually directly into a skip. This is a very efficient method. Otherwise, it is a case of wheel-barrowing tonnes of spoil round to the skip, taking time and money.

As mentioned above, the structural engineer will produce a method statement showing how to carry out the excavation. In addition to digging and propping with acrow props, the method statement shows the sequence in which to underpin the walls.

basement excavation

basement underpinning

In essence, the contractors underpinned the walls in 1m sections, each section being remote from the next to allow for setting. Sections numbered 1 were dug and underpinned first, sections numbered 2, second, and so on. Whilst each new bay was being dug, previous remote sections were left to set. 24 hours was specified for each section to set before dry-packing (ramming sand /cement /aggregate into any gap between the original wall and underpinning); then a further 24 hours before commencement of the next set of bays.

underpinning basement

In the photo above, you can see the underpinning being built up from the new foundation. Curiously, this image shows underpinning (with concrete block) of previous underpinning ! (concrete slurry) which had been poured in from a pit outside to shore up historical subsidence. The image below shows how new concrete blockwork has been built right up to the original wall; you can see where the original wall was above and below the earth level at this point by the change in colour.

significant underpinning for basement

In the photo below, you can see two substantial pillars underpinning the fireplace hearth in the room above. In our design, we looked to make use of every nook and cranny. The space between these pillars would accommodate the new condensing boiler. You can also see how new steels now span across the new walls to support the joists above.

underpinning for basement conversion; two pillars are supporting the fireplace hearth above.

To create more storage, we excavated beneath the rest of the house, but only to waist height to form deep cupboards. This saved money by limiting the work in these areas. We installed air-bricks in the cupboards for ventilation.

storage cupboards being formed at waist height in the basement

Basement wall construction

New basement walls had to support new steels and joists (you can see how the Edwardians supported the joists with loose brick piers in the photos at the top!). The construction drawings specified the reinforcement bars required to tie the structure together.

structural reinforcement

Tanking – waterproofing

There are several methods of tanking a basement. In this case, contractors fitted a cavity drain membrane as in the photo below – the back of the membrane looks like an egg-box. This method doesn’t try to stop water coming through from the ground, but any that does penetrate trickles down between the egg-box studs at the back of the membrane to a plastic channel, so protecting the inner face. The gradient in the channel carries the water around the perimeter of the basement to a sump. Often, builders will fit a pump in the sump to expel accumulated water to a drain. In the case study, water isn’t a big issue, so the sump simply has a cover and any water is removed manually with a scoop. This takes place less than once a year.

basement tanking

basement flooring

You have to dig deeper than you might expect, then build the floor up again to the finished level with the various layers specified on the drawings. In this project, the floor comprised 150mm compacted hardcore, a radon / vapour check membrane, 100mm mesh-reinforced concrete slab, 75mm insulation, 75mm screed (containing the underfloor heating), and then the floor finish itself which was polished concrete. So almost half a metre deeper than the finished floor level.

We installed wet underfloor heating in the basement on a separate zone, controlled by its own thermostat. We fitted the manifold remotely.

basement underfloor heating installed

natural lighting in a basement

Some basements may have a light well already, or will have one fitted as part of the conversion. Here, because part of the basement was just above ground, we installed windows to let in natural light. We also made good use of a half-glazed entrance door which borrowed light from the kitchen.

natural light for basement

Basement steps

We decided to form the basement steps on the exterior (within the kitchen diner) to maximise valuable space in the basement. The kitchen diner was large enough, so could afford to lose part of its footprint.

steps constructed to basement conversion

We tiled the steps with porcelain and installed step lights for dramatic effect. The glass balustrade is just visible on the right allowing light to flood into the basement through its half-glazed door.

step lights

basement fittings

Having designed the basement for its planned function, we specified locations for all the electrical sockets, lighting, home network, aerials and other fixtures. The contractors carried out their first fix, before they plasterboarded and skimmed the walls. We had also pre-fixed a large sheet of plywood rigidly behind the plaster that would take the weight of a large flat screen TV. You should try to think ahead to those heavy items that would benefit from more secure fixings.

Below you can see the metal studs to which the contractors would fix the plasterboard sheets. Also visible are the cables ready for their various lighting and power functions.

tanked walls

basement finishes

Specialists polished and sealed the concrete floor. We emulsioned the walls and ceilings simply in white to maximise the light. We designed and supplied the media unit and installed the home cinema and laundry.

polished concrete floor in the basement

How to excavate a basement

We hope this article has given you some idea of how to excavate a basement. As ever, planning is key. If you plan to excavate your own basement, have a clear idea of how you intend to use it, and plan the space completely; lights, sockets, everything. For a challenging project like a basement conversion, you should engage professional support. If you follow the processes laid out above, you will create a wonderful space, whether a games room, laundry, cinema room or swimming pool!

If you have a basement conversion in mind in the Bath or Bristol area and think we may be able to help, please read more about our home renovation services.

 

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