Basements - avoid the pitfalls

In heavily urbanised areas such as London and the south-east, going underground can dramatically increase the commercial value of a development. However, the complexities of basement design are often out-weighed by the risks that arise by over-looking the importance of managing impacts and relationships at surface level.

In our experience, clarity of communication and the confidence to challenge will prove invaluable to the success of a basement scheme.

Five crucial steps will help smooth the structural design process for a new or enlarged basement, and they apply equally to a luxury residential dwelling or a major commercial or public sector development. Following these steps ensures you attack the risks, eliminate wasted time and ultimately, save money.

It all starts with communication. Invest time in talking and you will save money. Communicate the philosophy of the basement – demonstrate how you have thought through the process of designing it. Talk to the temporary works designer and the contractor, ask them how they would build the structure and ensure they understand your vision and the importance of an integrated temporary and permanent works design. Make sure everyone involved is clear on the process and take advantage of 3D modelling to visually demonstrate the design and sequence of construction.

Thoroughly de-risk your project before you tender for construction. Conduct your research and close-out all risks. It is essential to close out all party wall awards at the design stage. Failure to do this leaves your scheme vulnerable to challenge and time consuming, costly revisions, a situation often exacerbated when contractors’ equipment is on site ready for use and then has to sit idle eating up your budget. Residual risks should be clearly identified allowing them to be closed out later, not forgotten. A high quality geotechnical engineer will also help de-risk a project by instigating appropriate site investigation works to understand what the foundations can withstand, and what the flow of groundwater is doing both below the basement and to the neighbouring properties. At planning stage, it is also vital to have in place all the relevant build-over agreements with third parties such as the utility providers and London Underground. Once you have achieved planning consent you don’t want any nasty surprises that affect your boundaries, impacting the design and the agreed construction schedule.

De-risking the project and communicating your needs will be greatly enhanced by regular site visits. Go to site and check the quality of the work. Working within space restrictions, as is often the case when building basements, can lead to a reduction in the quality of workmanship potentially putting at risk the water tightness of the designed solution. Visiting the site and working with the construction team reduces the risk of poor workmanship.

If you are managing the design of a site and a material change is made to the environment or the purpose of the basement, it is vital to challenge change. At an extreme level, if the use changes from car parking to habitable space, it immediately impacts the fire strategy, the MEP design and the water proofing. Has this been taken into account? In compact, tight spaces, the smallest of changes can have a huge impact on the build programme. For example, basements are sometimes designed to very tight tolerances to extract maximum value, so if a late decision to change the design or usage of the site results in a change to your design, this cannot always be easily incorporated. It could result in revisiting the party wall awards, the contractor needing to hire different equipment or changing the temporary works design. Therefore, it is important to revisit all of these issues thoroughly when a change is proposed and not just look at the implications on the design.

Finally, it is essential to understand the need for accurate reporting. This includes regular reporting on the quality and progress of work on site to ensure there is a written record of actions required, agreed by all parties, which protects you if issues arise. In addition, together with the architect, party wall surveyors and legal team, reports are often issued to fund monitors and/or investors which provide the trigger for the release of cash to deliver the project. Understanding the reporting sequencing that secures the finances for the project and communicating progress to your funders is as important as understanding the design requirements.