What to Consider when Converting a Basement in London?

As we know, additional space in a property is an extremely valuable asset.  Homeowners or purchasers are often looking at ways to increase the living space of properties they either currently live in, or, are hoping to purchase.

Much has been written about excavations to form new basements (and sub-basements) which have become increasingly popular in many areas.  However, lots of properties have existing basements and that space, which is at a premium, can be converted to add space to your house.

Some basements, such as the typical “cellar” beneath a hallway in a Victorian property can be successfully converted to create dry storage space.  Whereas others, of a larger size and where you may be wanting to create additional living space, can be designed as such and converted accordingly.

Before proceeding with such work, there are numerous factors which should be considered before deciding on conversion works, which include:

• Call in a specialist PCA (Property Care Association) Certified Surveyor in Structural Waterproofing (CSSW) and PCA Waterproofing Design Specialist (WDS) who will be able to help design a solution, which will ensure that your basement remains dry and fit for purpose.
• Depending on the objective i.e. are you considering lowering existing floor levels, increasing the size of the area by excavation, it is likely you will also need to engage the services of an Architect, a suitable building contractor and Structural Engineer, who will advise on party wall issues etc.
• Sub-ground level waterproofing works should conform to British Standard BS8102:2022.
• The CSSW is a specialist industry qualification which requires an understanding of waterproof systems, with continued professional development and training. As mentioned above, there is also a register for Waterproof Design Specialists (WDS.
• Preservation Treatments are pleased to say that currently two of our experienced Surveyors are on the WD Register.

Basement conversions do sometimes require planning consent. It may not be necessary; however, it is important you check this before starting any work. Back in 2015, a total of 1,968 planning applications for basement conversions were submitted to UK councils with the majority of these (1,497) being in London.  It is also necessary to have all the relevant building regulations approved before you get the actual work started.

When considering a conversion, you will need to let the insurer, who provides your buildings insurance know,.  It is likely they will reassess annual premiums, based on what the converted room will be used for, together with the effects on the structure of the property.

It is essential that suitable and safe access within the house is available.  If not, it will be necessary to create access that fits with the local building codes? Some local authorities require external access to be made available.

It will be necessary to ensure that all Health and Safety, party wall and planning authority requirements are met and neighbours are advised.

Depending upon the extent of work to be carried out – especially where this often involves lowering existing floor levels to create additional head-room height – this is where you will need to employ the services of a registered Structural Engineer before any work commences.  They will determine the nature of the soil, underground structures and often, also look at similar developments in the area.

As a result of basement excavations, nearby properties can be affected by the displacement of the ground water levels and pressure.  Since the 1980’s water levels have steadily increased which has resulted in flooding of sub-surface infrastructures in some areas.  This is particularly so, where road levels are raised, rebuilding of existing properties is carried out, along with extensions and re-routing of drains/sewers/water pipes etc.  This can result in diversion of natural underground streams, into sub floor voids or basements, which do not have the added protection of a Structural Waterproofing System.

London has dozens of hidden underground rivers and tributaries. Many of these were hidden during the 1800’s due to pollution by waste.  Although these rivers are no longer polluted, they are still running under properties in London.  You should ascertain whether or not there are any existing underground rivers close to your property before commencing work, as this is likely to have an effect on the system to be specified.