House Extension cost

Building an house extension, Sheffield, 2001


No two houses and customer requirements are the same and so we cannot give a price based on a simple question like "How much does a two storey extension measuring 9m x 4m cost?"

Important: It is strongly recommended that you read on to learn why such questions cannot be answered by any builders without having both plans and having seen the site even for a ball park figure.

A short lesson about foundations

Clay dig

Strip foundation is the most common type of foundation used to build a new building or house extensions, because they offer good strength for their cost. In order to lay down strip foundations you need to excavate a trench at a minimum depth of 1.0m and have solid undisturbed ground. Yellow clay is the indicator to signify that you have reached solid ground.

Once those criteria are met, your builder would then invite the building inspector from your local building control for a site visit where they can check out that your builder has met those criteria satisfactorily. If all goes well, you will be granted permission to lay down the foundations.

Remember, 1.0m is the minimum depth but there are no guarantee that when the builder starts to dig that they will hit solid ground at that depth. They could well end up digging much deeper. For every extra 1.0m dug you need another skip to shift the sub-soil away. You also need to order more bricks for the build just to get it back to ground level. All this require more labour time. As you can see the deeper you go, the more money it costs.

Your builder could end up with the kind of scenario where they will have to stop digging because it becomes uneconomical shifting massive quantities of earth, trying to reach that solid (yellow clay) ground which may never materialise. They would have no option but to resort to pile foundation, which is the ideal solution to building on top of poor sub-soil. Pile foundation is far more expensive than using a strip foundation and often require specialists to carry out this kind of work.

The point of this short lesson is to demonstrate that when it comes to pricing foundations it is far from being simple.

How do you price up foundations for your house extension?

When you get your quote from your builder, each stage of the build should be itemised with a price so you know the breakdown in what you are paying for.

It would be itemised like this:

X rows of bricks to DPC (Damp Proof Course) level
Dig for foundation at 1.0m deep
Strip foundations
Skip hire for 1.0m soil removal
= ?
= ?
= ?
= ?

If it deviates from this, then this is considered as additional unforeseen costs which could not be priced up at the start of the project.

This is a typical example of why you need a contingency amount of 10-15% set aside for such scenarios.

If any company was to give you a fixed price for foundations then they are either inexperienced or are going to charge you close to the cost of the worst case scenario in order to cover themselves, which is not good for the customer should it end up being a simple 1.0m dig.

Word of warning: If it was the absolute worst case scenario for the builder and they were to honour a fixed cost for foundations whereby they ended up being out of pocket, then you need to be aware that since it is at such early stage of the build you could not be sure that inferior materials or cheaper labour would not end up being used in order to recoup some of this loss.

Surroundings that impact on the price of house extensions

There are many external factors that can impact on the price that customers don't think to take into other consideration when asking "How much does a two storey extension measuring 9m by 4m cost?"

I will highlight some which clearly have a massive impact on price.

House extension with cellars

If you have a cellar or your house extends close up to the edge of another property which has a cellar then what you really should be asking is:

"How much does a three storey extension measuring 9m by 4m cost?"

Even with the cellar being below ground level does not automatically mean it is possible to lay down strip foundations above cellar level in order to build your proposed two storey extension. The reason for this is that there is a slim risk from pouring tonnes of wet concrete into a trench which could exert enough outward pressure for the whole thing to collapse into the cellar void since this is the weakest point. The risk of this happening is very slim but one that is deemed unnecessary and so the building inspector from your local building control will not authorise it.

Retaining walls

It is not only extending out to other buildings you have to worry about. If you extend right up to the edge of a retaining wall that is holding back several tonnes of earth then you can't go digging a trench right next to it. To do so would weaken the footings and could potentially cause the wall to collapse which is likely to end up causing a landslip. The solution to solving this problem can often end up costing a lot of money depending on the retaining wall in question.

Restricted access to building site

If access to site is limited whereby your builder cannot use machinery such as a digger to dig the trench or that access is restricted whereby materials can only be ferried by hand, then additional cost for labour needs to be took into consideration. A typical example of this happening: You live in a terrace house which are common in Sheffield and that the access to the rear is restricted.


If you have drains where you want your extension building then these need to be bridged. If you have a manhole cover where your extension is being built then these needs to be moved. Cost of this varies and again depending on whether or not it needs to be bridged or moved taking into account of access for digger.

Other surroundings that can impact price

  • Large trees with roots nearby
  • Slopes of the site
  • Moving existing services


You may not have much choice when it come down to the price of the materials. The price of these are governed by what the original building has been constructed in. The perfect match is always the best option.

Things to consider:

  • Masonry units

Prices of masonry units varies depending on what you use which is down to what your existing house is using. Common engineering bricks are far cheaper than using natural stone. The amount of masonry unit needed is down to knowing how many windows and doors or even a garage door you plan to have. Issues of reclaimed bricks and toothing-in the brickwork impacts overall costs.

  • Roofing

A flat roof is far cheaper than having a pitch roof. Decision to use pitch roof over flat roof are down to being maintainance free for many more years and that extensions with pitch roof are more sellable which also gives better returns when you do decide to sell your house in future. Materials used for the roof also depends on your existing roof material. Slate tiles and clay tiles vary in cost depending on where they originate from.

  • RSJ (Rolled Steel Joint)

These vary in cost greatly depending on size used and based on how much you wish to open up the extension with the main building. You may want to open up the entire house and if there is a chimney breast in the way which needs removing, then this complicates things and adds to the overall costs.

Geographical location

The cost of living in London is far more expensive than in Sheffield and so you should take into consideration the cost of labour and materials based on where you live if you find some figures on the internet.

Fixtures and fittings

Once your extension is built, you have new rooms to fill. Did you want to include the cost of buying and fitting a new bathroom and / or kitchen for your extension?


While some of the above may not apply to your particular planned house extension, a builder without seeing plans and the site will not know which of the above applies to your particular requirement when you asked "How much does a two storey extension measuring 9m by 4m cost?"

Despite not having answered your question in terms of giving a ball park figure I hope the above has given you food for thought as to why such a question cannot be answered, even for a ball park figure, without going into much greater details as to what you actually want and why a site inspection is also an a necessity.

If you are in need of some further advice regarding house extensions from an experienced builder in Sheffield, including advice on applying for planning permission, then by all means contact us and we will do what we can to help.

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