Whether designing a retrofit basement for the first time or planning your 100th project, there is still a myriad of questions to be answered. After all when it comes to subterranean architecture and design each basement project is as unique as the building it’s designed around. To help make things easier, we have put together a useful list of the most common questions architects ask about retrofit basement shells.

How Much Does a Retrofit Basement Cost?

The average cost of a basement in London is generally between £2,500-3,500 per square metre. However this does differ depending on an array of factors such as whether you are digging externally, what materials are required for the structure, electrical wiring, legal expenses (for planning permissions, etc.) and damp prevention requirements.

Is it Possible to Retrofit a Basement?

Everything is possible within reason. Obviously, if the property is a modest terraced home and the aim is to install a 30ft basement swimming pool, this would be unrealistic.

Considerations when planning a retrofit basement include ground level, soil type, water concentration, foundations, surrounding houses. There will also be some limitations on the overall design, therefore the best course of action is always be to consult a knowledgeable basement specialist to determine the possibilities based on the existing structure.

Can I Dig Externally?

The short answer is yes, however when building externally, digging deeper and adding more piles keeping the structure stable and secure is vital. Additionally, retaining walls are required to keep the soil retained.

Another point to consider is that waterproofing is even more crucial than usual with external subterranean structures, as when new ground is excavated it often contains a great deal of moisture.

How Many Piles Do I Need and Where Do I Need Them?

The amount of piles needed all depends on where the basement will be. Externally, more will be essential for the retaining walls. However, if building directly under an existing underpinning (the process of strengthening the foundation of an existing building or other structure) is required so this should be calculated by a structural engineer and distributed at critical load points.

How Does Waterproofing a Basement Work?

Waterproofing is one of the most important elements in retrofit basement construction. The most common and reliable technique of basement waterproofing is by lining the inner walls with textured Polyethylene membrane sheeting called a Cavity Drain Membrane. Cavity Drain Membranes are also vapour proof and offer additional insulation benefits as well, making them the most popular option for waterproofing a basement.

What are the Most Common Basement Planning Issues?

Planning permission usually takes around eight weeks to approve and can often take longer, so if the schedule is tight, this will need to be accounted for in the very initial stages of the basement project. It’s crucial to consider as well, that there are several other types of permissions you’ll likely need to apply for in addition to planning, such as party wall agreements, street works licences, parking suspension and skip licenses.

Whatever the requirements of the basement project, Knowles can draw on their over 20 years experience to assist you at every juncture. To find out more about our highly specialist services, contact us.