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Getting to the bottom of it

With land in high demand, basements are common on new build and redevelopments alike in London, but many schemes are presenting problems during construction. However, new planning guidance for such developments being adopted by some London boroughs may not be the hoped-for panacea.

Basement construction in London really started to hit the headlines at the end of 2011 when the Health & Safety Executive (HSE) closed down a number of sites due to unsafe building practices. But that was not the end of the story for the sector that has defied the recession by continuously growing over the past five years.

Loft conversions were once de rigueur for making the most of your home, but in areas where land costs are high and space scarce, such as London, attention has also turned to basements. Basement developments are now common on new-build properties - both domestic and commercial - as well as for existing properties, where adding basement levels is growing in popularity.

“The system calls for the site investigation and structural design to be carried out before the initial planning application, which is a significant financial burden for smaller, speculative proposals.”

Nick Langdon, CGL

One of the London boroughs affected by this underground trend said that it saw a four-fold increase in planning applications for basement developments between 2003 and 2012 with no sign of the rate of increase slowing.

Basement

There are concerns that councils are using BIAs to block basements

In a bid to ensure applications for new basements are well thought out and planned, Basement Impact Assessments (BIAs) are gradually being adopted by some London boroughs. But according to CGL director Nick Langdon, while the basic concept itself and ambition is good, the actual documents are something of a blunt instrument and could be creating new problems for the councils.

“BIAs call for all basements - from those that involve dropping a floor by a foot or so to give an existing cellar enough headroom to be used through to large, multilevel new domestic basements and commercial developments - to go through the same planning process,” says Langdon.

“The system also calls for the site investigation and structural design to be carried out before the initial planning application, which is a significant financial burden for smaller, speculative proposals.”

Carrying out the site investigation so early is also potentially disruptive for redevelopment schemes that may not get the go ahead for some time and also take some time to reach the construction phase.

The BIA concept was born out of guidance developed by Arup for the London Borough of Camden, which has been published under the document name of CPG4, and is now in the process of being taken up by other councils, such as Kensington and Chelsea and Fulham, in the capital.

BIAs aim to create a structured approach before the development is submitted for planning to help ensure that all factors have been considered before the application is made.

“We have seen some BIAs for adjacent properties that present conflicting evidence. I am not sure councils are critically reviewing the evidence”

Richard Ball, CGL

According to Camden’s CPG4 document, the purpose of the BIA is to “assess whether any predicted damage to neighbouring properties and the water environment is acceptable or can be satisfactorily ameliorated by the developer”.

“The process is split into five stages and starts with screening and scoping of the project before moving onto a site investigation and study stage, which includes questions regarding issues such as party wall issues, ground water flows and ground movements associated with the work,” says CGL engineer Richard Ball. “The fourth stage is an impact assessment and the final stage involves the council providing feedback to the client with a review of the document.”

It is the client’s responsibility for developing the BIA, but many are assisted through the process by the contractor or architect. Camden calls for the document to be prepared by someone who “holds qualifications relevant to the matters being considered” and for the ground stability element to be carried out by a chartered civil engineer or a geotechnical specialist as defined by the Site Investigation Steering Group.

Nonetheless, both Langdon and Bell raise concerns over the quality of some BIAs being used in planning applications and also question the ability of local authorities to carry out a rigorous checking process. “We have seen some BIAs for adjacent properties that present conflicting evidence,” says Ball.

“I am not sure that the councils are critically reviewing BIAs and are just accepting the statements if they are prepared by someone with the right qualifications.”

Although BIAs are designed to address the technical issues of basement construction and avoid potentially damaging developments from going ahead, there is concern in the sector that some councils are using BIAs as a method to block planning on basement developments. This is driven more by concerns over the number of basement schemes rather than the potential damage some of them may cause.

Kensington and Chelsea is one London borough that is looking at the approach of applying a blanket ban on multi-level basements and those that cover the full garden area. “This is based more on issues related to disturbance to neighbouring properties and maintaining trees in the gardens to preserve the character of the borough than technical limitations,” says Ball.

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Carrying out structural design ahead of planning is not an easy task from engineers

Langdon adds: “In reality a poorly conceived single storey basement can be far more disruptive than a well-designed two-storey basement.”

Given that the basement industry has grown considerably during the recession and demand for this form of construction looks set to continue rising, CGL believes that a new approach is needed.

“Introduction of BIAs may put the brakes on growth in the basement sector for a while but it is a sector that is likely to continue growing,” says Langdon. “Basement construction is a very sensitive area due to how it affects people’s homes and also the fact that some of the structures involved are sensitive themselves due to age, so some kind of planning tool is essential.”

With regards to an alternative approach, Langdon suggests that maybe the system could be refined with different levels of BIA depending on the scale of the development.

Nonetheless, it is likely that the councils will be trying to keep the process in the pre-planning stage to make planning approvals simpler. “The need for the structural design work to be carried out ahead of the planning application goes against the government’s ideal of streamlining planning processes in the UK too,” says Langdon.

Trying to carry out the structural design ahead of planning is never going to be a simple solution for the engineers involved. “Trying to predict ground movements is at the top end of geotechnics and even the most basic of basement developments needs the same level of knowledge as a major commercial basement,” says Langdon. “This cannot be carried out without detailed information about the ground conditions.”

Whatever the answer is, Ball and Langdon’s colleague, business development manager Andy O’Dea, believes that the geotechnics sector has a key role to play in moving the issue forward. “As an industry we need to find a voice to discuss BIAs and find a way to improve guidance and ensure there is rigorous assessment,” he says.

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