It is not surprising that more and more UK residents are asking, ‘Can I build a basement on my house?’. We hope this article will give you some of the answers you need for that question. Considering the limited space we have for development in the UK and the stringent planning laws it is a shame that more houses weren’t and aren’t being built with a basement, as is standard in countries like Australia and the US. Basements can increase the size of a house by up to 50% without needing more land, and although planning is required the planning objection of over-developing the footprint of the site is mitigated.
Modern basements don’t need to be uninviting spaces fit only for storage thanks to modern building techniques but they can be a comfortable attractive floor for a gym, bedrooms, home cinema or to remove more functional rooms from the main house like additional bathrooms, storage and laundry. One of our long-term clients even had a basement big enough to house an indoor cricket net.
There are three grades of basements, 1-3. Grade 3 is defined as dry, and generally termed as a habitable environment and we will assume for this paper that we are dealing with Grade 3. The design of a basement is fundamentally about understanding and managing the risks of such a structure to ensure it can be built successfully and won’t present problems in the future.
If you’re thinking about adding a basement to your home, there are three important elements to understand before you can make your dream a reality, each stage informing the next;
- The feasibility of your plans before applying for planning permission
- The best construction process
- The expected costs
Feasibility
Structural Engineers will help you determine the feasibility of your plans by considering the impact of the following 4 considerations.
- Geology
- Ground Water
- Heave to counter uplift
- Location
It may seem obvious but understanding the ground that you intending to dig for your basement is crucial. Understanding geology, ground water, heave and location and their implications are key to the question of – Can I build a basement under my house?
Geology
Understanding the geology of the site is crucial in the early stages and it will inform on the best type of construction method to use for the project. The type and nature of the soils beneath your home/garden will have a considerable impact on the choice of construction technique and form of the basement.
Digging into bedrock is going to be time consuming and expensive for example. Whereas excavating into water bearing gravels will require careful consideration as to how to control water levels within the excavation without compromising the stability of the existing foundations.
Ground Water
The enemy of all basements. Understanding the water in the ground under your home is critical and we always recommend that a site investigation, which includes long term ground water monitoring is completed.
BS8102:2009 is the Code of Practice for Protection of Below Ground Structures Against Water from the Ground. BS8102 says you must design for a full head of water, i.e up to ground level unless long term ground water monitoring demonstrates that a lower level is appropriate. This is why SWJ recommend that long-term ground water monitoring is undertaken as it allows us to design to the actual ground conditions than an assumed worst case.
Why long-term monitoring? A site investigation carried out in January would typically show a certain level of water, whilst the same investigation in July would yield a different result owing to differences in seasonal rainfall. Any design needs to consider the potential of water all year round and how sources of ground water such as the inflow of surface water (from rain to burst pipes) and water pressures on the retaining wall and below the slab base will affect the structure.
The presence of water is significant, particularly where people are trying to use lighter modern methods of construction such as timber and lightweight steel because there isn’t the self-weight to resist any potential uplift or pressure.
Heave to counter uplift
There are two sources of uplift:
- Hydrostatic Pressure
- Heave due to removal of overburden pressure.
Hydrostatic uplift or the uplift pressure exerted by water in the ground is a crucial thing to consider. Essentially if you bury anything in the ground, if it’s not heavy enough the water pressure will lift it, push it upwards, towards the surface. As already discussed, the presence of water may cause problems with the project and even cause a collapse during the early stages of construction. Any measures used to temporarily reduce the ground water table during construction will at some point have to be turned off, so the question is whether the mass of the new basement is heavy enough to resist the upward force caused by water pressure? Your structural engineer will prove by calculation that the forces will remain in balance and floatation will not occur.
Water pressure is the most common sauce of uplift, which is why it’s the first point to understanding whether you can have a basement or not.
“Heave due to removal of overburden pressure” is a rather posh way of describing what happens when soil that has been compressed for millennia by the weight of the soil above is removed. Imagine the state of the washing up sponge that has been accidentally left beneath the casserole pot that was soaking overnight. It’ll be in a sorry state come morning and a fraction of its former cross-section. By lunchtime, with the weight of the pot removed it will have recovered its former volume, the same happens with soils but at a much slower rate.
Once you know the pressure you’re resisting it’s about understanding the weight of the intended construction method and how you can add weight to resist uplift. Basements with lightweight forms of construction, such as timber and light-weight steel frames can be an issue. Architects and construction companies might not fully understand the implications of using these materials, they may be more focused on the economic or sustainability factors of these materials rather than their ability to resist uplift.
Adding weight to the construction to resist uplift, your three options.
Option 1 – Weigh it down by extending the base slab
Extending the base slab of the basement’s foundation mobilises the weight of the surrounding soil. This is not always possible or straightforward depending on the boundary and the location of the basement.
Diagram 1. Extended base slab
This can be an appropriate solution for heavier forms of construction i.e. traditional construction but can be uneconomic with lighter more modern forms of construction such as SIP panels.
Option 2 – Nail it down with concrete circular piles
The second option is to use piles to resist the uplift and nail the building down to the ground below. The most common form of piling used for this purpose is the Continuous Flight Auger technique. The CFA method pumps liquid concrete into the hole as the auger extracts so the ground is continually supported during installation.
Piling is a more robust solution for dealing with uplift, but it’s slow to install and significantly more expensive. You must consider attendances, getting the ground ready to have such large machinery on site. Precision timing is also required because the concrete must be ready at the right time to maintain its workability and ensure a reinforcement cage can be inserted to the correct depth before the concrete cures.
Video 1. Continuous Flight Auger method.
If you have to choose one method of piling over the other (circular v sheet) and the cost isn’t a factor it boils down to speed. If your site/location is dry enough, and you don’t anticipate any problems with uplift then sheet piling is a quicker and more efficient solution.
The uplift pressure acting on the underside of the slab is significant and needs to be dealt with when using tension piles to resist the uplift. In order to minimise the thickness of the base slab, a void is formed beneath the slab to allow the soil to expand into thereby eliminating the pressure due to heave.
Option 3 – Calculate and allow for movement
Granular soil recovers instantly but clay soil takes years to recover. There is nothing that can be done to stop its recovery, as discussed above, you just need to relieve the pressure by forming a void or design for it.
The final option is one that is only generally suitable for new buildings where the basement is beneath the full footprint of the buildings. We accept there will be some movement/rise and take that into account for the other elements of the build. These issues would include ensuring there is enough flexibility in the underground drainage and incoming services to accommodate the movement and paying attention to threshold details.
We would not recommend this solution where the basement abuts or is partly under the building owing to the potential for damage through differential movement.
Location
Understanding the amount of access needed to achieve a basement in the location you require is crucial, particularly if you’ve neighbours and boundaries to respect. You can’t just dig someone else’s land or assume they will give you access. Location and the access it affords can dictate the construction method for the basement which will have an impact on timings and costs. Below we look at several scenarios of location, access and possible solutions.
Diagram 2. Access for a receding slope of the earthworks
Batter Slopes
With basement location image A above, you need a lot of land for access and to consider how to maintain unimpeded access to the rest of your property. The safest way to dig for a basement is to batter the slope, have a receding slope of the earthworks for easier and safer access.
This removes the failure wedge, earth that could potentially slide into the excavation. The situation is like building a sandcastle on a beach. Once the sand dries it isn’t able to resist the lateral load and collapses sideways. The same failure can occur in excavations and this risk needs to be eliminated.
As a rule of thumb, the batter slope would be dug at 45° but that is dependent on the ground. If the ground is particularly soft or is a combination of water and sand (a bad combination) you may have to dig a shallower shape which could lead to you crossing a boundary.
Diagram 3. Batter slope
Underpinning because of limited access
If the location of your proposed basement is footprint C (see below) you don’t have space to batter slopes, so underpinning is an option.
Diagram 4. Restricted access to site
Underpinning is essentially a hit and miss sequence, where you dig under existing foundations in stages every three days. A 1m dig with 4m in between. I consider this method time consuming and unnecessarily risky.
Diagram 5. Underpinning – Diagram courtesy of The Constructor.org
There are other ways of supporting the existing foundations that do not require people to be excavating underneath the existing building, such as the piled walls described above (option 2 Concrete circular piles)
Underpinning part of an existing building can also lead to issues with differential settlement as we are now creating a hard spot beneath part of a building
Differentiation of movement because of proposed basement location
Diagram 6. Basement plan that exceeds the existing building’s footprint
Basements are not always built directly under the footprint of the existing buildings. Basements that will exceed the existing building’s footprint make the design more complicated.
The original house’s foundations will sit on roughly (depending on age) 450 – 900mm below ground. This ground will be softer and as it’s closer to the surface and its consistency will be affected by seasonal changes. A basement’s foundations are typically 400mm deep, and at that depth, the clay tends to be stiff. This layer is hundreds of thousands of years older than the ground the house’s existing foundations sit on and it has been consolidated by the weight of the soil above.
The different strata of foundations where the basement is under half the house is a recipe for cracks. If the basement is under the house’s footprint this is not a problem as any movement would be even across the existing house’s footprint.
Temporary Works with sheet piles to protect neighbouring properties
Excavations for basements has been liked to open heart surgery. If you need to protect your neighbour’s property, then you can build a temporary wall using sheet piles. My opinion is that temporary works should form the basis of a permanent build as there seems little point in adding something, just to remove it again.
The rule of thumb with sheet piles is that you pile (submerge) 2.0m into the ground for every meter you want exposed above ground, so for a 4.0m deep dig, you would need a 12.00m pile – 8.0m embedment and 4.0m to be exposed.
Sheet piling enables a temporary (or permanent) wall to be built to protect your neighbour’s property enabling you to build a basement close to a boundary.
With sheet piling there are the material costs of the sheets to consider and the cost of installation using a silent piler – see video below.
Video 2. Demonstration of silent piler
Removing the piles is also a hard thing, it requires cranes because of the height and clearance needed to remove them from the ground. This can also result in further complications with oversailing your neighbours’ property and so on.
This is why we recommend that the working space is not considered temporary but becomes the permanent wall.
Applying for planning permission
Planning authorities set the requirements for any basement planning application and these can vary across the local authorities, but you will find them on their websites. However, every authority requires a Basement Impact Assessment (BIA) to be submitted in support of any new basement planning application. There is no standard template for a BIA as each authority may have specific inclusions. Planning authorities usually publish templates on their websites for a more standardised review process.
Regardless of the varying inclusions required within BIAs each one needs to be specific to the proposal. The collation of all the information we have discussed above will contribute to an evaluation of the direct and indirect implications of the proposed basement development. This is both a factual and interpretive report that includes;
- Underlying geology and groundwater.
- The impact on drainage, sewage, surface water and ground water.
- Ground movement due to the proposed development
- Burland Scale – The Burland Scale categories damage and is based on ease of repair of visible damage. It is the internationally adopted methodology that is recognised and used by the Building Research Establishment and the Institution of Structural Engineers
This is why physical site investigations are necessary to establish ground conditions, and survey existing structures and foundations using trial pits and/or bore holes.
Summary. The process and considerations of a basement
Basements are a great way to add space to a house on a restricted site, whether the restriction is planning permission or space. The key to a successful build is research and preparation, there can be no shortcuts. Problems and shortcuts could spell disaster during construction or worse still a leaking basement.
We recommend anyone considering putting a basement under their home follows this process.
- Decide they want a basement
- Talk to a structural engineer and architect at the same time. You don’t want designs from an architect that promise a pleasing aesthetic but aren’t feasible because of a restriction the architect wouldn’t be aware of. Similarly, you don’t want to engage a structural engineer on their own as they will only design you a water-tight boxlike construction; Functional, but miserable.
- You should all meet on site to ‘walk the course’ and discuss solutions
- Your Architect can then provide you with a realistic solution
- At this point, it wouldn’t hurt to get a rough estimate from a quantity surveyor so you know if you can realistically afford to complete the construction before going tender
- Once you’re happy the build is potentially within your budget, submit plans to planning
- Once planning has been approved then seek proper quotes from specialist basement construction companies
- We recommend you get an independent project manager involved, be it your architect or specialist firm. Basements are a difficult thing to get right. Making the right decision at right time is vitally important and if this isn’t done the homeowner leaves themselves vulnerable to additional costs and claims, as they won’t understand the implications of making the right decisions at the appropriate time.
The one thing we haven’t covered in this paper is pricing. Pricing for basements is difficult as it depends on the client, but we will look to address this in a future paper soon.
If you require any further advice please do not hesitate to give us a call on 01993 225 085 or email rhys.skym@swjconsulting.co.uk
