Basement and Modern Extension on Historic Coach House

MJ Rooney built this basement onto the rear of this period coach house in Blackheath and then constructed a modern extension on top with a reinforced concrete superstructure. This basement was for two additional bedrooms onto a basement courtyard and a home cinema and basement gymnasium.

House Extension in London

This house extension in Blackheath, unlike most London basement projects, had a wealth of space for an outdoor swimming pool, pool house, and a summer house. Because of this, mass concrete underpinning was the chosen method. This would help to create the space for a basement gym, basement cinema room, and two basement bedrooms. We started with having to underpin the existing historic coach house for the basement conversion. The project included a two-story extension above the main house with terracotta cladding and a zinc roof. The lines of the modern zinc roof along with the warm terracotta cladding added style and class to the already beautiful red brick coach house.

Grand Designs basement extension timber joists floor

Underpinning a Basement

The underpinning process for the basement conversion started with trial holes to find out the size of the footings. Mass concrete underpinning was the chosen method. After removing the existing floor, we excavated shafts under the footings and filled them with concrete, forming pins. MJ Rooney installed the pins following a designed sequence to avoid any unsupported parts of the structure. London basements tend to be tight on space which would require smaller reinforced concrete pins.  

The underpinning process supports the house as well as forming the walls of the basement. After the completion of all pins, we excavated the whole basement space. We put all the drainage for the basement bedroom en-suits and for the cavity drainage waterproofing system. And then MJ Rooney prepared the floor to receive the cement blinding, the waterproof membrane, and the steel reinforcement. Finally, we poured the concrete slab and formed the concrete basement box.

Waterproofing a Basement

The waterproofing in basement conversions starts with the quality of the concrete. The joints in the underpinning need to be clean and lined with a bentonite strip that swells when in contact with water. We had to mix properly and compact well the concrete itself to reduce any passages for moisture into the building. We also lined the basement slab with a bentonite waterproofing membrane to stop water from coming through the slab and wall-floor junctions.  

The main waterproofing system for the basement conversion consisted of a cavity drainage system. This involves applying a dimpled drainage membrane to the floor and walls. If any water comes through the walls it will travel behind the membrane to drainage channels and out to the basement pump station. This way no water can ever come in contact with the internal finishes.  

This basement conversion had a lightwell which also drained to the pump station. The clients chose the delta waterproofing pump stations. As well as the pumps they also come with a WIFI monitoring system. It tells delta when the pumps are running and if something is wrong. The system will contact the client or a pump engineer in the case of an emergency.

Grand Designs basement extension terracotta steel cladding ensemble

Cladding a House Extension

The basement conversion extends out from the existing house providing footing for the new extension. We picked concrete-formed walls and floors for speed, strength, and the ability to have large openings for glazing. Having years of experience with concrete construction, it only makes sense to bring this construction method above ground. After having cured the concrete, we struck all formwork to prepare the structure for its finishes.  

The house extension in London requires a level of design that will sit comfortably in its surroundings. Balancing this with the modern spec of the clients’ requirements was delivered by the use of terracotta cladding and a zinc roof. The terracotta cladding mirrored the red brick of the house. Its modern design elevated the humble material. The zinc roof stood out as a statement piece, the uniform color and sharp lines literally capped off a stunning extension!

Five Basement Conversion Ideas

Five basement conversion ideas to get the most from the space in your house!   

  1. Lighting

    Easily the most important part of the design is the lighting. From morning lighting to engage the brain and get that sunny feel to evening mood lighting. In a space with no windows, it is important to be aware that we need to design all of the atmospheres in the space!  

  2. Lightwells

    Lightwells work very well to give natural light into the basement. The bigger the better! A light well you can walk out into will defiantly transform the basement bedroom and remove the enclosed feel that many have.  

  3. Size

    Low ceilings are defiantly a no-no! Obviously, you want as much room to move around in as possible but high ceilings also give the feel of space. In the grand scheme of things going cheap on the depth of the basement could destroy any want to be in it! Even the word basement has claustrophobic connotations so let’s do the best we can to make you forget that it’s even underground.  

  4. Environment

    Environmental controls are a blessing in spaces with no natural airflow. The design needs to integrate heating and air conditioning as a whole. Not only will it make the space comfortable but it will also stop damp! In a basement, you are the creator of the environment so you need to make sure it’s perfect for you and the structure alike.  

  5. The contractor

    This may seem obvious but when it comes to contractors, DO YOUR HOMEWORK!  We often take perspective clients to previous projects just to get a feel for what they could create. Do not go on price alone! If you do, at least make sure it’s a like-for-like comparison. Ask for a comprehensive quote breakdown to see what is missing from one but not the other. One may be cheaper but doesn’t have half the experience, what will that cost in the end?  

Check the swimming pool that we constructed in a basement below an existing house located in the Kent countrysides.

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