To help you help you estimate the costs to carry out a basement construction or conversion project, we have put together a series of Articles which we believe may assist you in doing so. As a result, we have also compiled a list with costs you would expect on a full scale basement project which requires underpinning in the London Area.


Construction Costs (STRUCTURAL)

Without a doubt, a basement conversion or the build of a new basement would have a great impact on both your home life. Furthermore, this would also add great value to your property when taking into account the rising house prices and cost of moving. This is applicable especially in the London area where house prices are high and moving to a larger house is costly. Although not always cheap, a basement conversion can add an entire extra floor of living space to your home. Therefore, you can choose to build a basement under the footprint of the existing property or you can extend this under your rear garden. Moreover, you can also add a front and rear light well to the basement. In the below article we will try to cover and estimate a typical basement cost in the London area:

Already have a decent sized basement ?

In case you already have a decent sized basement under the footprint of your property, the cost for a basement conversion will be less costly. This is mainly due to:

Basement drawing showing foundation levels reported to basement conversion cost

Existing Basement/Cellar foundation levels

The foundations you already have will deepen below the existing basement level, therefore the underpins to be installed will be shallow. As a result, this will provide you with a great saving on the overall project cost.

The soil to be excavated and removed will be considerably less

Finally, as you contractor will not need to spend as much time on your site, the preliminaries (such as the day by day running costs of the site including licences and parking suspensions) will be greatly reduced.

Overall, for the installation of shallow underpins and waterproofing, you could expect a cost starting from £1,600.00 + VAT/sqm.

No existing basement under the footprint of your property

Basement conversion cost calculator showing no existing basement with shallow foundation levels.

No existing basement/cellar, shallow foundation levels

In this case, the costs for constructing a basement will be higher as opposed to having an existing basement for the simple reason that the underpins to be installed will need to be deeper. Also, the soil to be removed and excavated will be more in volume. As a result, in this case, throughout the London area you can expect costs to start at £2,400.00 + VAT/sqm excluding design costs and related professional fees.

Professional Fees

Architects and Structural Engineers Fees

Architects Site Survey, Architectural Design including existing and proposed drawings – £2,000.00 upwards dependant on size of project

Planning Application Submission – £206.00

Structural Engineers Site Survey, Structural Design, Calculations and RC Details – £3,500.00 upwards dependant on size of project

Structural Engineers Method Statement and Temporary Works Design – £3,000.00 upwards dependant on size of project

Party Wall

Party Wall Awards to include serving of notices and preparation of party wall award. Also, this will include the visit to site to inspect the adjoining owners property to produce schedule of conditions- £1,800.00/ per adjoining owner dependant on the involvement required

Checking Engineers. It is now a commonly used practice to employ a checking engineers during the party wall process to check and sign off the design on behalf of the adjoining neighbours. It is hard to estimate the cost because this is charged on a time spent basis. Usually, costs will start from £800.00 dependant on the size of the project as well as the quality of the designing engineers design.

Building Control

Building Control (Private) – £1,400.00 dependant on the scale of the works

Soil Investigations

Because building a basement requires extensive excavation works, your local authority may require you to carry out a soil investigation before to determine the soil conditions and components. Consequently, the results from these tests will influence the structural engineers design and the materials and method employed to build your basement. Also, the investigation will determine the ground water table, particularly important for the structural design, construction method and waterproofing design. The cost for such a soil investigation and analysis, dependant of the level of details requested and ground conditions found, can start from £1,600.00 + VAT including laboratory testing of soil samples.

The cost of a basement conversion can vary significantly. Costs can depend on the particulars of your property and other factors which need to be considered during the design. As a result, we need to note that the above costs are representative only. The quoted amounts represent typical charges which a basement company in London would charge.


Construction Costs (FIT OUT)

In general, for the fit out stage you can allocate an overall cost starting from £1,000.00 + VAT. For the reason that, this stage very much depends on your preferences as far as internal finishes go, this amount can increase considerably.

Due to the fact that the fit out works are not as specialised as the basement construction, you would have the option to issue these works for tender to finishing contractors only. Also, you might have a preferred builder who you wish to use. Therefore, your chosen basement company or contractor will cary out the excavation and basement structural works. Once the structural stage is completed, the basement contractor will hand over and liaise with your fit out builder for them to continue with the fit out works. However, there are benefits of having one basement company completing your project from start to finish, including fit out works.



As a general rule with all major construction projects, as a safety factor, you will need to allow for a contingency sum based on the scale of the works. This can be anywhere between 5% to 10% added to the overall budget. This will cover you for any additional works and variations. Although reputable basement companies will give you a fixed price, it is inevitable that some additional works will need carrying out. These can be works which are either not included in the design or you would want to add elements during the works.


In the below case study, we are going to summarise the above notes and draw a conclusions in respect of the costs.

Basement Location Hammersmith and Fulham Borough
Existing basement No
Proposed new basement floor area 90 square meters
Front light well 4 square meters
Rear light well 6 square meters
Proposed finished ceiling height 2700mm FFL
Soil Conditions No obstructions, No contamination
Project duration 34 weeks structural + fit out
Soil removal soil collected in skip at front, removed by grab
Adjoining neighbours 2
Architects design and drawings including planning £4,800.00 + VAT
Structural design and calculations £6,600.00 + VAT
Soil investigation £1,600.00 + VAT
Party Wall (est. checking engineer fees £1,200.00) £7,200.00 + VAT
Private Building Control £1,400.00 + VAT
Preliminaries (site setup, licences and related) £23,000.00 + VAT
Structural works £235,000.00 + VAT
Fit out works (including 2nd fix items and materials) £135,000.00 +VAT
TOTAL £414,600.00 + VAT
Est. Contingency sum to be allocated: £33,000.00 + VAT


In this paragraph, we are going to point out a few elements which should be taken into consideration by you, as the beneficiary. Firstly, please bear in mind that the only way to truly find out a cost for your project is to invite a basement company to provide a cost for the works. Secondly, as a general rule, contact at least 3 companies to provide you with a preliminary quote for the works. This will ensure you can make a valid comparison between quotes. These days most of the companies offer a no obligation property survey and tender for your basement project. Thirdly, during tendering, set out as clear as possible what you want to achieve as the initial tender would be based on this. Also, you need to bear in mind that any changes in the design mean changes in the overall tender figure.

Finally, once you have made the decision that a basement extension is the way forward for you and you have assessed the feasibility for such a project, your next step is to look for a company or contractor. Therefore, please read browse thru our website where you can locate a company who will be able to help you. Furthermore, you can also search for professionals who can assist you throughout your project.

The above costs are representative only as the cost of a basement conversion can vary significantly dependant on the particulars of your property and other factors which need to be considered during the design and construction. Furthermore, the quoted amounts represent typical charges which a basement company in London would charge. As a result, Basement Company Reviews does not give any warranty or other assurance as to the above calculations, its accuracy, completeness, timelessness or appropriateness for any particular purpose