Guide to Basement Extensions in London

Basement Architects in London

Shape Architecture has built a reputation as experienced basement architects in London. Our experience spans all types of basement design from double basements in Kensington, to numerous basement conversions in Fulham alongside whole house refurbishments, as well as basement extensions in Kensington constructed below mews houses. On this basis of this knowledge we are able to set out what we believe are the ingredients for a successful basement project. These range from the principle issues of light and space, to points relating to the design and construction process of a basement, and the many issues of detail that arise in these projects thereafter.

A Guide to London Basement Extensions

Shape Architecture has recently produced a brochure detailing ten basement conversions in London that we have designed and built over the last three years and this is available for download on our website . The brochure details a wide range of basements from single storey to double storey, those located in conservation areas to Mews locations and in Cul-de-Sacs. This breadth of experience enables us to suggest those issues that we have found to be important in designing successful basement extensions in London which we list below.

Basement Extensions: Key Design Issues

Five tips in designing a basement extension:
1. Internal Height
2. Location of the Stair
3. Type and Number of Lightwells
4. Artificial Lighting
5. Storage

Tip 01 – London Basement Conversions: Internal Height

The floor to ceiling height of a basement extension is an important decision. Our basement conversions in London vary from 2.75 metres to 3.5 metres. We would say that 2.75 metres should be considered as a minimum height, whilst a figure around 2.9 metres will feel spacious. The feeling of space in a basement project cannot be gauged simply by a floor to ceiling calculation however; a significant factor will also be the number, size, and location of rooflights and other sources of natural light. The proportions and size of each room and the way in which rooms are accessed all play a part as well, alongside colour schemes and the use of artificial lighting. Floor to ceiling height is a good starting point though.

In respect of the technical issues relating to basement extensions in London, one should also be mindful of the level of the water table. This is a technical consideration and can be simply addressed. A soil survey should always be undertaken at the planning stage to a depth of the desired internal height plus the lowest part of the excavation required to form the foundations. Most planning applications will require this analysis as part of the Construction Methodology Statement that forms part of the planning application.

We have designed a number of basement extensions in London where the basement sits below the water table. The approach to the structural design remains largely the same. As noted above the key is to have a full understanding of the ground conditions at the design stage in order that an appropriate design is pursued.

Tip 02 – London Basement Conversion: Location of Stair

The location of the internal stair connecting the existing ground or lower ground floor is very important. Not only is this the means by which the basement extension is accessed, it is also very often a design feature in itself as well as being a means by which natural light can be brought down into the basement. We have explored a number of different locations for stirs in our portfolio of basement extensions in London. For example in one of our Fulham basement conversions we have located a stair in a large void within an open plan kitchen dining space. Located over the void is a large rooflight and this provides a great deal of light into the heart of the basement and views of the sky from the basement. In another Fulham basement we have located the stair in a glass cube located at the garden edge in order that the basement stair and its enclosure interact with the garden and provide again a great deal of daylight into the basement alongside views of the garden. Other London basements that we have designed and built have stairs that follow the exiting run of stair flights. In various Kensington Mews basements we have also integrated stairs in lightwells.

Tip 03 – London Basement Conversions: Type and Number of Lightwells

Lightwells can provide light, air, views out, an exit out, and an amenity space. Clearly, they are an invaluable part of every basement conversion and can define the character of the basement. At Shape Architecture we have also designed basements without lightwells that introduce other means for light to enter the basement. We will describe that method later.

Common to many London basement extensions is the garden lightwell. Typically at the basement level a series of sliding doors or bi-fold doors give onto the lightwell. At ground floor level the lightwell is then crossed by a link from the ground floor that provides access to the garden beyond. The type of sliding/folding door is an important choice and this relates to the width of the glazing frame. Narrower frames can be much more expensive than a ‘standard’ 50mm with.

Our basement conversions in London have explored the connection between the interior and the garden beyond the lightwell in a number of ways. These vary from glass bridges with glass balustrades to more significant glass platforms incorporating patterns of clear and semi opaque finishes to the glass. Often at the far ends of the glass platform a steel grille is incorporated for ventilation to the lightwell below. We have also linked an elegant timber and glass stair to a glass bridge in a basement conversion in Fulham. At other basements in London a series of steps lead from the lightwell up to the garden. These steps provide easy access to the garden, as well as an elegant feature when a glass balustrade is applied at garden level.

Tip 04 – London Basement Conversions: Artificial Lighting

A basement extension will benefit greatly from the addition of simple and elegant artificial lighting. Many of our basement conversions in London have featured lighting set around the internal stair, in the garden lightwell, and around feature rooms such as the wine store to great effect. Successful lighting schemes marry efficient lighting with several elements of feature lighting such as an elegant pendant fitting, with the lighting of several architectural features such as the internal stair or a glass bridge. It is also often successful when natural light sources such as rooflights have discreet artificial lighting set into the well of the rooflight.

Tip 05 – London Basement Conversions: Storage

Basement extensions offer a great opportunity to add additional storage to a property. These solutions can range from large storage rooms built under the garden or terrace, to various stores located off the basement circulation space. In several of our London basement conversions we have built storage rooms giving off the lightwell built under the garden which have been constructed as one large room or two rooms with separate access doors. The basement corridor linking the garden to street side accommodation provides an excellent location for various store rooms.

A Guide to London Basement Extensions: Further Issues

London Basement Conversions: Fire Safety

As basement specialists in London, Shape Architecture is experienced in designing basements that comply with fire regulations. A typical issue is the desire for a contemporary open plan series of spaces, with stairs that are located in reception rooms. In both cases these design features move away from the traditional fire rated layout of a hall with stair, and then a series of cellular rooms set off the hall. An increasingly common solution is the use of domestic sprinkler or misting systems. In our Fulham basement conversions we have often located a water storage unit for the sprinklers system in basement store rooms. Alternatively an upgrade to the water supply pipe entering the property can provide the required water supply.

London Basement Conversions: Access to the Garden

In basement extensions with a garden lightwell, access to the garden is achieved both at basement level and at ground floor level. At ground floor level the garden is frequently accessed over a glass bridge or platform that spans the lightwell. From the basement lightwell a stair can be introduced that connects to the garden. This can vary in design from stone steps set alongside the garden wall to modern timber and glass constructions that scale the side of the lightwell and can connect with the glass bridge. Our basement extension in Fulham at Ellerby Street illustrates the stair connecting to the bridge and the simple and elegant detailing that can be achieved. Alongside the physical connection between interior and garden is the visual connection between the two. This can be a glimpse from a reception room located at the front of the house to large expanses of glass in sliding doors giving onto the garden from the kitchen/dining space.

London Basement Conversions: Other Extensions and Refurbishment

Our basement extensions in London are on the whole carried out alongside a variety of other work in order to reduce future disruption and maximise the available space. This can include a complete redesign and refurbishment of the ground floor over the basement with side and rear extensions. At higher level basement extension projects in London often have loft conversions and pod rooms added.

London Basement Conversions: Energy Efficient Design

At Shape Architecture we have experience of designing and building energy efficient buildings over the last fifteen years. This experience enables us to suggest a range of simple energy efficient measures from the planning and layout of the building to the use of natural light and the specification of low energy fittings.

London Basement Conversions: Interior Design

In our basement extension projects we often provide the interior design services. We also work with a range of interior designers and can recommend appropriate ones if required. Where we provide interior design services our role varies depending on the clients requirements. It can range from a ‘supportive’ one, where the client has strong ideas of how the interior should look like to one where we are providing a range of design options to discuss with the client. We are particularly experienced in designing bespoke furniture, ranging from free-standing and walk-in-wardrobes, to bathroom vanity units and library and desk designs for a study. Our drawing packages always have detailed elevations drawn of every wall to insure the accurate setting out of wall finishes such as tiling and various other fixtures and fittings.

London Basement Conversions: Garden Design

Shape Architecture is happy to provide concepts and detailed design for garden spaces. As with interior design we also work with a range of garden designers and can make recommendations accordingly. The relationship between the ground floor and the garden beyond is particularly important. A variety of our basement extensions and whole house refurbishment in London have dealt with this issue in a variety of ways. In a number of basement projects in Fulham we have located elegant stairs, both internal and external at the point at which the garden begins.

London Basement Conversions: Bathroom Design

Our experience of designing and specifying bathrooms both in basement extensions in London and many other projects is vast. We are able to draw on this experience to design well planned bathroom spaces that are simple, elegant and filled with light. As noted in various sections above we provide detailed setting out elevations of every wall in the bathroom. We also have many documents that we have put together detailing a wide variety of finishes, fixtures and fittings that we make available to our clients.

London Basement Conversions: Kitchen Design

Having completed many basement projects with whole house or additional upper floor refurbishment, we have designed many kitchens for our clients. In the first instance this will involve the designing of the location of the kitchen and its relationship with the spaces around it. We will develop this with our clients using 3D computer graphics. The process will also consider the layout of the various kitchen appliances. It is quite common at this point for a client to engage a kitchen designer or kitchen company to progress the detailed design of the kitchen. We will continue to liaise with the kitchen specialist as required.

London Basement Conversions: Utility Room Design

Utility Rooms to basement conversions in London tend to be located at a mid point in the basement extension, off a short internal corridor and easily accessible from the stair into the basement. As with the design of bathrooms, we have a great deal of experience in designing simple and efficient utility rooms and can provide our clients with many sample layouts.

London Basement Conversions: Technical Design of Basements

The technical design of a basement conversion is particularly important given the many technical issues involved in this type of building project. The structural design and design of any temporary structure is very important, and it is vital to have a structural engineer who is experienced in this type of project. Equally important is the relationship between architect and structural engineer in ensuring that the building structure and building fabric are well co-ordinated prior to a start on site. At Shape Architecture through the large number of London basement extensions that we undertake, we enjoy good and well established relations with a number of structural engineers.

Keeping water out of a basement extension is equally important. We have encountered numerous different situations in this regard ranging from single storey basements that sit below the water table and double basements that sit considerably below the water table to typical single storey basements sitting above the water table as well as those that have water courses running through them. In all these instances there are well established structural and fabric design approaches to deal with the issue and an experienced design team will be able to design the basement accordingly and draw on their experience to talk the client through the process.

The issue of thermal compliance in basement extensions will concern the insulation of the building envelope to include insulating the basement floor. Equally important is the specification of the glazed elements that meet or exceed the requirements of the building regulations. The acoustic performance of the basement very often concerns the acoustic separation of the basement from the floor above. There are very many acoustic insulation products that can be put within the basement ceiling void or else within the floor make-up itself to address the issue of structural borne sound, which might be from someone walking overhead. Service pipes can be lagged in acoustic wrap and holes and gaps sealed with acoustic sealant or spray foam. Many issues relating to noise though can be addressed at the design stage in respect of the location of various types of spaces.

A further technical issue of much importance is the design of the strategy for fire escape. Many basement extensions and the floor above are designed as very open spaces. Doors and dividing walls are removed to provide a sense of space, more light and a better physical and visual connection between spaces. This approach does raise issues in respect of fire escape. A traditional layout of a home has a hallway with a stair in it and doors off it opening onto a variety of rooms and this layout is repeated on every floor. By this method a fire protected route is created. If these spaces are opened up and the fire rated divisions removed then a sprinkler system should be considered. At Shape Architecture we work with a number of sprinkler companies and have installed many sprinkler systems in our basement extension projects. As an alternative to sprinkler systems a mist suppression system can also be considered. Water pressure into the home has to be addressed and this may mean the upgrade of the water supply pipe into the property or the installation of a dedicated supply tank.

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