Although there is no actual formula to calculate the cost of underpinning for your project, please see below which elements can have an impact on the underpinning works and overall project cost:
Proposed Basement Ceiling Height
The proposed ceiling height in a basement project will play a major role when it comes down to overall budget. A decent and most commonly built ceiling height in a basement is 2700mm. However if you opt to increase this you will find that the cost will significantly increase exponentially.
Typical Underpinning with reinforcement
If you already have a decent sized basement or cellar under your property, this can significantly reduce the overall cost of your basement construction price:
1. The majority of the cost on a basement project lies with the underpinning and excavation works. If you already have a decent sized cellar/basement under the footprint of your property, the cost of the excavation, removal of soil and labour should decrease accordingly. Furthermore, you will also save on the period of licences and parkings suspensions.
2. In case of an existing basement, usually you will find that the existing foundations are deeper. These will likely extend to formation level of the existing basement or cellar. This will have a large impact on the cost of the underpinning works. This is due to the fact that as the shallower the underpins, the more you save on costs.
3. You will also save on the period for which licences and parkings suspensions need to be obtained for. During the basement works, you/your contractor will need to obtain various licences. These can include skip licence, hoarding licence, scaffold licence, conveyor belt licence as well as parking suspensions. The mentioned licences are quite costly to obtain. As a result, the lesser the period required for basement excavation and underpinning works, the lesser the cost for licences will be.
Has any of the adjoining neighbours carried out either a basement dig out or underpinning works to the party wall ? If so, your party wall will be most likely already underpinned (depending on the actual design – which you can look up in the party wall award). Consequently, you would be able to make use of the installed underpins. In this case, the cost for underpinning a basement would be considerably lower. However, you need to take into consideration the fact that there might be a requirement for you to make a payment to your neighbour for making use of the installed underpins as set out in Section 11(11) of The Party Wall Etc Act 1996. This will usually be based on 50% of the current cost of underpinning.
On numerous projects it has been found that concrete has overspilled to the adjoining neighbours property during the underpinning works. For this reason, it might be the case that the basement contractor will charge you for cutting back concrete overspills. The process of cutting back concrete can expensive and is usually charged to you. Furthermore, it will need to be carried out carefully whilst making sure that the vibration does not travel thru the party wall and cause damage to structures. However, the cost for these works you can reclaim from your neighbours thru the Party Wall Agreement. You can also find more information on the Party Wall Process at https://www.gov.uk/party-walls-building-works.
Existing Ground Floor Structure
This detail is of relevance in case of a new basement floor under the property. The new floor structure would need to be able to act as a suspended floor on its own. If the floor structure is a concrete slab, this will most likely require replacement or extensive structural steel works to support from below which can turn out costly. In case your floor structure is constructed on a wooden joist structure, this will probably not require removal but installation of structural beams under.
In case that your neighbours have already carried out a basement conversion, it might be worthwhile to ask them about the soil conditions. This detail will be assessed by an intrusive site investigation (boreholes), investigation which can be costly. As a result, it would be of great assistance if you can obtain a preliminary idea on this element. The structural design and construction is very much based on the soil conditions. Hence it is worth looking into this at early stages.
Water Table Level
The water table level is assessed with a site specific soil investigation. In case this exceeds the formation level (your basement slab level), you will most likely need a dewatering system in place during the works. The cost for this can vary dependant on the size of your project and the water inflow.
Access to your property
A project of this scale requires extensive logistics, movement and parking of large vehicles. These can include soil removal (grab lorry, skip lorry), material deliveries on a 7.5 Tonne HGV and large drum mixers for concrete pours.
Main sewer diversion – if you know of any main sewers running within the boundary of your property, it is likely that this will need diverting. As a result, this can add to your overall project cost.
Intended use of the basement
Important to the overall basement conversion cost calculation is what you intend to use the basement for. For example, a foul water pump will be necessary if you are thinking of having a bathroom or a kitchen, . This will deal with the foul water discharge from the basement. This is in addition to the ground water pumps. These deal with water build up behind and below the installed cavity membrane. Your Architect will be able to advise on these design elements.
Basement conversion costs
For a specific cost calculation and case study, please visit our Basement Conversion Cost page.