11 Jun 2019

Top tips for basement conversions

Basement conversions are becoming a popular method to add floorspace and value to a property. However, the decision to tackle one should not be taken lightly.


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A project that seems so simple can, if not executed properly, present complex challenges. Mistakes made during the design and build process are extremely costly and the build will, no doubt, impact on your daily life. You only have to look at the recent court case revolving around a botched basement conversion to see the acrimony getting it wrong can cause.

As such, a clear plan of action from the outset and an acute understanding of the role your suppliers will play in the construction journey is essential.

Where should you start? We asked a panel of construction experts to share what they think are the top considerations for self-builders and renovators when approaching this type of project:

Surveying the setting

Before you start digging, you need to know what’s in the ground below. This could include public infrastructure. For example, any sewer on your property connected to a neighbouring property is considered public and you’ll need the local water company’s approval to build near it, which may mean reconfiguring your basement.

You need to take into account any nearby trees, as they’ll have radial root systems extending to a distance one to one-and-a-half times their height. Building Control have regulations regarding how close to specific tree species you can build, so this has to be checked.

If you have neighbours adjoining you and a shared ‘party’ wall, a signed party wall agreement will be needed before work starts.

Jeremy Wiggins, Technical Director, gpad london ltd

Design for success

Without a good structural engineering design, the works may devalue the property and prevent Building Regulations approval being granted. This should be seen as a necessary requirement before the construction journey begins.

Furthermore, this design needs to take account of the working method, so that the structural safety of the property (or neighbouring properties) is not compromised in the temporary case, as well as the permanent case.

It’s also important to understand the different skills offered by professionals involved in the build. For example, do you know the difference between a structural engineer and a chartered surveyor, when they should be consulted and for what service? Do your research before the project commences to avoid potentially costly and stressful problems at a later stage.

John M Staves, CEng FIStructE, Fellow and Vice President of the Institution of Structural Engineers

Material values

Looking at the finer details of thermal efficiency is crucial if you aspire to build to the best sustainability standards. This will save money on energy bills, as you can minimise how much ongoing heating or cooling your home requires.

Masonry products are highly reactive materials and deliver homes which are warm in the winter and cool in the summer, due to their inherent thermal mass properties. Despite this, you should still look out for thermal bridges; these are areas of a build with a higher thermal conductivity than the surrounding materials, typically breaks in insulation. They can account for roughly 30% of a building’s heat loss.

At the CBA, we offer a free and easy-to-use tool to help anyone involved in a building project determine the ideal materials for their project. The U-value calculator measures the thermal transmittance of a building’s exterior wall and enables its user to achieve optimal energy performance. The CBA website also has a wide variety of thermal bridging details available for free download.

Christopher Stanley, Housing Manager, Concrete Block Association

Further information....

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