House Extension Cost

Having an extension built is the ideal solution to compensate for the lack of space in your home. Here is our complete guide which will allow you to better understand the construction work, what goes into determining the final cost of an extension, and how to go about estimating it. Hopefully, by the time you reach the end, you'll have an accurate, near-complete picture of the cost of building an extension.

How Much for House Extension: What Type of Extension are You Considering?

When it comes to expanding a home, there are many types of extensions homeowners consider: the construction of an additional room, a separate annexe, or a room elevation. Each of these options has advantages and limitations and is suited to the needs of different homeowners. Extension costs by type vary a lot.

Furthermore, the prices for the construction of an extension vary greatly depending on the surface area of the structure, the materials used, the complexity of the project, the construction time, and the regulations in force. This is why the best way to consider the extension costs is different from one type of extension to another, and that's why we've created subsections of the most popular extension types.

Extension: Additional Room

The construction of an extension allows you to create an additional room and increase the living space of your home. This type of project can be carried out in two ways: by extending the existing structures horizontally or by altering the roof and adding a room vertically.

In such construction projects, the new structure must be integrated into the existing building. Count around £1,000 per m2 for a wooden house extension, £2,000 per m2 for a steel and glasshouse extension, and £1,500 per m2 for a cinderblock house extension. In all instances, you must keep a few things in mind:

Find out about the feasibility of the project according to local laws and legislations (we'll explore this further ).
Find out about the permit application process if necessary.
Compare quotes from different building professionals
Plan out the project precisely to avoid unpleasant surprises, cost overruns, etc.

Horizontal Extension for Rooms

The construction of a horizontal extension is one of the most common solutions. The floor area of the house is increased, and it does not involve major modifications to the rest of the house. The extension can maintain the same style as the existing building or seek to break out from the norm by appealing to an offbeat or more modern design.

One of the disadvantages is that it is necessary to go through structural work involving earthworks, the creation of foundations, masonry, and roofing. Without forgetting that it will be necessary to create an opening in the existing building to connect the two living spaces. The usable area of the garden will be reduced as well.

Vertical Extension for Rooms

In a vertical extension project, the living space will be increased without modifying the foundation of the housing. The roof elevation requires reinforcement of the frame, masonry work to raise the walls as well as the review of the insulation of the walls and the roof. Note that the slope of your roof must be less than 30 degrees even after the extension.

Take into account that it will be necessary to install a staircase and that will occupy a floor area of approximately 4 m2 per floor. Count between £1000 and £1900 per m2. Make sure that the neighbourhood construction and planning rules of your municipality do not limit the height of your home.

Extension: Veranda

The construction of a veranda extension project is, relatively speaking, a more affordable investment. Depending on the nature of the materials used, the price range extends from £400 to £1,800 added to the extension costs per square metre. This type of project lends itself to many variations: it may be possible to install electricity, heating (for year-round use). Here is some information about the prices of the professionals you are likely to call on:

Electrician: the average hourly rate of £35 to £45
Carpenter: the average hourly rate of £40 to £60
Painter: the average hourly rate of £25 to £40

Note that there are also verandas in kit form (prices ranging from £2,000 to £12,000). Although, we won't explore this kind of extension anymore in this article.

Do not hesitate to request several quotes to obtain the most suitable service for your project. Comparing the quotes to our cost estimation software is a quick and good way to learn whether you're being overcharged or not. Professional construction companies should help you define your needs, take precise measurements, and study the types of finishes and construction methods that can be used in detail, taking into account the specifics of your home and the type of climate.

Extension: Annexe

Separate annexe construction projects can come in a variety of styles, ranging from prebuilt sheds without water or electricity starting from just £5,000 to high-end outbuildings costing around £12,000. As the assembly of these annexes can be tricky, we strongly advise you to contact an experienced professional who will be able to quickly turn your project into reality.

As with all construction projects, the choice of materials is the major factor determining the number written on the final invoice. It is important to consider the following points to come up with an accurate guesstimate:

Are you planning to purchase a prebuilt extension?
Are you planning to use custom plans (it may be necessary to hire an architect)?
Are groundworks necessary? (£15 to £25 per cubic metre)
What architectural style are you planning to use? Is it the same style as the rest of your home or is it different?

Cost Factors

Don't neglect the additional costs when planning your work. Keep in mind that the more ambitious is your extension construction project, the higher the costs will be. Some costs are unavoidable:

The fees of an architect can quickly represent between 8% to 15% of the total cost of the work. Plan around £1,500 for a medium-sized project.
Costs of excavation, demolition, and site preparation are sometimes required. Count from £50 (rubble wall) to £200 (reinforced concrete) per square metre for wall demolition work (depending on the material).
Count £80 to £200 per cubic metre of concrete if it is necessary to pour foundations.
In the case of the creation of or an update to a house framework, the prices oscillate between £50 and £90 per square metre.
Count £40 to £80 per square metre for roofing with bitumen shingles with metal flashings. Metal and slate covers are high-end options.
You can also consider installing exterior vinyl siding for an average of around $ 5 per square metre.
The installation of plasterboard in the interior as a finish for the walls and the ceiling costs between £20 to £45 per square metre.
Insulation will cost you between £15 to £30 per square metre depending on the type of insulation you choose.
Depending on the material and model, prices for an interior door start from £35 for an entry-level wooden door, up to several thousand pounds for a high-end bespoke door.
Likewise for windows, count from £85 for an entry-level PVC window to £1,600 for a bay window. The choice of single or double glazing will also have an impact.
For obvious safety issues, we advise you to use the services of a professional electrician for all electrical work (£35 to £45 per hour).

Legal Considerations, Hiring an Architect and Cost of Extension

Consult the local authorities to study the feasibility of your extension construction project according to the rules established by your municipality. They'll determine the area that can be built on your land. You must also take into account the distance from neighbouring buildings as those are also regulated and might affect extension costs.

If your accommodation is near a historic monument, a building architect must validate the project and specify any limiting conditions as there are laws and regulations in place protecting these monuments.

Likewise, if you are planning a major overhaul of your interior, an architect will be able to advise you appropriately and suggest arrangements that you would not necessarily have thought of on your own. Your project will be more solid and interesting, and the architect has the know-how required to work around the convoluted British laws.

Lastly, keep in mind that property and housing taxes will increase accordingly. This is part of the additional costs that need to be included in the extension costs and not to be forgotten as it can be substantial.