House Extensions

House Extensions

Baileybridge builds a great many extensions in many different styles.  We advise on all of the design aspects and use your vision to create drawings and take these through the Planning and Building Regulation process before carrying out the build.

There is more to building house extensions than a lot of people think. Below we have shown the key build stages and explained a little about what is involved with each, in an effort to enlighten anyone thinking of embarking on such a project. It might also be worth reading our Do I even need a Loft Conversion or Extension? page.

Our approach is to arm our Clients with as much information as possible so they can envisage what is involved and can plan how they will live around the work. If you’re having a kitchen extension or your kitchen is being renovated, it is important to rig up a temporary kitchen for example.  We have had a lot of fun setting up temporary kitchens in garages, bedrooms, sheds etc. to enable Clients to suffer as little inconvenience as possible during the period the work takes to complete.

Why not get in contact with us with your ideas for your project and see if we can help.

The Stages

Groundwork – ie digging the foundations: A major consideration is access for heavy plant because without this the labour costs will escalate.  What will you do with the earth that is removed?  A modest extension will result in 10m3 of excavated earth and disposal of this by skip or grab lorry is very expensive at about £50 per m3. Sometimes we use this to landscape the garden which is especially helpful if your land is not level, as it can be used to build up any sloping corner.

Soil pipes & inspection chambers: Quite often an extension will be built over the site of existing soil pipes and inspection chambers (drains). This will necessitate new inspection chambers being installed as these are not allowed to be left under buildings. There is a Building Regulations requirement to be able to rod between chambers therefore adequate rodding points will need to be installed – i.e. new chambers. If your extension involves a kitchen or bathroom, whilst the groundwork is taking place the plastic pipes for this waste will be installed where these go underground.

Foundations: Minimum foundations will be at least 1m deep and 450mm wide and can be much deeper or wider depending on ground conditions, the proximity to trees and whether the extension is single or double storey. These have to be filled with concrete which costs around £120.00 m2 plus equipment to move it from the mixing lorry, plus labour.

Brick and block work: A modern extension will have a double skin (wall) of usually blocks on the inner skin and either bricks or blocks on the outer skin, depending on if it is being rendered. It will have a cavity between these skins of 100mm. 50mm of this will be filled with insulation, leaving a ventilation space of 50mm.  Your finished walls are therefore a minimum of 300mm – a little more allowing for plaster-boarding inside and plastering.

Below ground drainage: If the underside of the floor falls below ground level then an oversite (floor made from concrete) has to be installed with a drainage pipe with a non-return valve, to remove any water that may get under the floor (rain water). A minimum 200mm gap has to be left between this concrete oversite and the underside of the building’s floor. This is so that in situations of flash flooding, which is a more common occurrence these days than it used to be, there is no chance of the water coming up through the floor.

Rainwater soakaway: The rainwater soakaway is something not everyone is familiar with but current Building Regulations state that a soakaway must be installed to accommodate any new building’s rain and surface water including that from the below ground drain. Guttering is installed on the building which connects to a rain water downpipe which connects to a 100mm ground pipe that runs into a soakaway.

A soakaway is a ‘big hole in the ground’ and its size depends on the roof space of the extension but the minimum size is 1m2. It is filled with ‘crates’ and its purpose is to allow the water deposited in it to ‘soakaway’ into the ground around it.

A soakaway must be located no nearer than 5m from the dwelling, any other buildings and the boundary. It usually entails a trench being dug across the garden where the 100mm pipe is positioned, which goes into the big hole. This is infilled with earth and if relevant new turf is laid and within no time there is no trace of it. The downside is these are relatively expensive and depending on ground conditions, circa £2000.00.

Even if your existing rain water is deposited into the main drains, this is no longer permitted for new work. The reason is that if these drains flood, their contents can back-up into the dwelling and potentially foul waste will fill into your sink, bath, shower etc. which does not really bear contemplating!

A soakaway with crates installed:

House extensions: A soak away with crates

Breaking into the existing dwelling: There is no hard and fast rule about when we break into the existing dwelling; it depends on so many things relating to the build.  We leave this as long as possible in order to afford the Client as minimum and as short an intrusion as possible, as once this happens, they are in no doubt they have a building project taking place! Breaking in usually involves propping the existing dwelling’s ceiling with acrow props so the external wall can be removed and then the steel beam or beams have to be very carefully maneuvered into position and cemented in.

Work in the part of the property the extension connects to: When you are considering the costs it is important to think about not only the extension costs, but also those of remedial work to the rooms it joins onto. The chances are you are going to be removing a fairly substantial bit of external wall and inserting a couple of steel beams.


This is likely to mean that the room this affects will need significant refurbishment as well as the extension itself. Here an existing room with the steel ‘goal post’ being installed.  We also show this same space finished. You can see how disruptive the extension is to the existing room. This may seem obvious but we have had Clients ask us if they can continue living in this room whilst the work takes place. Understandably it is difficult to envisage the project if you have not been involved in one previously.

When Baileybridge provide pricing we always include the completed project – i.e. any rooms that will be affected by the refurbishment.

Floor: Very often the floor is made from concrete block and beam such as shown in the picture and diagram below.

extensions floor

Roof: There are many options with roofs and Baileybridge will always design the most suitable for your extension whilst liaising with you as to your preferences.  You can have a flat roof, a pitched roof, a mono pitched roof (lean to). The pitch can have a gable end, a barn end, a hipped end etc. A roof will comprise of its final covering – i.e. slate, tile, felt, lead, fibreglass, EPDM (rubber) etc., a layer of breathable felt if it is a tile or slate roof, usually 200mm of insulation under the felt and then plasterboard and plaster. The roof is supported by timber joists and rafters, sometimes with a number of steel beams. A flat roof might have a lantern light or two, a pitched roof might have a number of velux windows. All of our roofs are hand made by our skilled team, on site and are totally bespoke.

Guttering & Downpipes, soffit and fascia: These are the finishing touches to a roof and can be made from a variety of materials. Many gutters and downpipes are PVC but products made in cast aluminium, whilst expensive, look fantastic and are much more suitable for use on historical properties.

PVC rain water goods, soffit and fascia
Cast aluminium rain water goods with timber soffits

Services: As the structure is created the services will be installed, e.g. electric cable, hot and cold feeds, gas, waste etc. They will be installed in the correct position and ‘first fixed’ – i.e. electrical back boxes will be installed, valves will be put on pipes awaiting 2nd fixing to radiators, sinks etc.

Plaster-boarding, dry lining and plastering: To form the room inside the structure both the ceiling and the walls have to be plaster-boarded or dry lined and then plastered. Some might argue that if you plasterboard or dry line well, you do not need to plaster but we always want a perfect finish and this is not achieved without plaster, no matter how good the dry lining is! Dry lining is where the plasterboard is fitted with the use of adhesive, as opposed to screws. If the internal walls of the extension are blockwork then the plasterboard will be fitted with adhesive (dry lined).  Where internal partitions are made using timber studwork, the plasterboard is screwed to this stud work.

Joinery: Typically joinery includes door frames, architraves, skirting, window cills etc. Once the plastering is completed the joiners will carry out all of the timber based work in readiness for the decorator.

Decorating: Decorating is often over looked as being very important, with some Clients saying they will undertake it themselves. This is fine if you are an expert decorator but many people are not, like the creator of the chocolate cake below! Along with plastering, decorating it is one of the most important parts of the build or refurbishment – the icing on the cake – and you want the icing to look fabulous!


Obviously all parts of the build are absolutely necessary and no one would like it if their walls were wonky, their building subsiding or their roof leaking. However if you have a visitor they are unlikely to say – ‘what a fantastic roof, it does not even leak’, or ‘goodness aren’t your walls straight and no sign of subsidence’; but they will comment on the overall finish and what they actually see is the decorating.

Walls should have a perfectly even orange peel effect, ceiling to wall joins should be totally straight and certainly not bleeding into each other, the woodwork should be smooth and not have any drips or ‘bobbles’ and all surfaces should feel smooth to the touch.

extensions finished