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​How can I plan and cost my single-storey extension?

Johannes van Graan Johannes van Graan
Canonbury House by Jonathan Clark Architects Мінімалістичний
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Improving the functionality and style (not to mention visual appeal) of one’s home can take many forms. It can be as simple and quick as repainting a wall or adding a few potted plants to the porch, or it can be a project that involves much more time and costs like adding a single-storey extension.

The latter option is a perfect way to create extra legroom in a home while also transforming your living spaces to better suit your lifestyle. But obviously a project of such magnitude needs to go hand in hand with some decent planning, especially considering it might cost a pretty penny.

So, before you set out to buy new furniture for that extra bedroom (or kitchen or bathroom or living room) you’re planning, let’s take a detailed look at the cost factor of your single-storey extension.

1. How much will a single-storey extension cost?

The final cost will depend on a number of factors, including the results you’re after. For a rough estimate (and based on a finished cost per square metre), we’ve included the following figures: 

• Basic quality £1,380 to £1,680.

• Good quality £1,680 to £1,920.

• Excellent quality £1,920 to £2,160. 

The costs will further fluctuate depending on the rooms within that single-storey extension. For example, with a kitchen, we recommend that you budget from around £4,500 to £16,500, depending on the specification; if you go high-end, costs can increase considerably.

2. Doors and heating

сучасний  by Phi  Architects, Сучасний
Phi  Architects

Rear Extension

Phi Architects

Of course that extension will just be so much more practical if it has large doors that open up onto the back garden. And here we present two options: bi-fold doors and sliding doors. If these sound like must-haves for you, factor in £1,400 to £1,800 per linear metre.

It may also be possible to extend your existing central heating system. Otherwise you could just opt for underfloor heating. Should you require a new boiler for your heating demands, the estimated cost of a gas boiler replacement is around £2,300.

3. Professional project fees

Kitchen Rear Side Extension, Norwich NR2 3BL by Paul D'Amico Remodels Сучасний
Paul D'Amico Remodels

Kitchen Rear Side Extension, Norwich NR2 3BL

Paul D'Amico Remodels

Let’s not forget about the design of that extension you’re dreaming of. Budget for design fees from 3 – 7% of the construction cost, with a minimum of around £2,400 to £3,600 for the plan drawings. 

Construction drawings will usually cost the same and you might require a structural engineer to size roof joists and foundations. Factor in another £500 to £1,000.

4. What about Planning Permission?

light cube by homify Мінімалістичний

light cube


In England, the planning fee for a residential extension is £172. A certificate of lawful development is £86 and the fee for discharging planning conditions is £28 per request. Bear in mind that other reports may be required, such as tree report (£720 upwards), flood risk assessment within flood zones (£720 upwards), etc. 

When planning a more ambitious extension, you will need planning permission for factors like using materials that differ from the original style of the house, and if you plan on building a balcony or raised veranda. 

To save both time and costs, ensure that you go through your plans thoroughly with an architect or builder who is familiar with the local planning authority and their preferences.

5. Building regulations

External Photo - Doors by Trombe Ltd Сучасний
Trombe Ltd

External Photo – Doors

Trombe Ltd

It would seem that size does matter, as the fees for building regulations approval will depend on the dimensions of your extension. Plan from £250 for one to 10 m², and £900 for 80 to 100 m². This is relevant regardless of whether you go for a full-plans application with everything approved before you start, or a building notice application where you need only give 48 hours’ notice of commencement.

homify hint: If your neighbours consent formally to your extension, you won’t need a party wall settlement, which can save you some costs. If not, having a surveyor arrange party wall agreements for you can cost from £700 to £1,000 per neighbour.

6. Finding an architect or designer

Canonbury House - 1 by Jonathan Clark Architects Мінімалістичний
Jonathan Clark Architects

Canonbury House – 1

Jonathan Clark Architects

Of course the design fees of these professionals vary from company to company. But in your hunt for the ideal expert, you need to search for someone whom you can talk to easily, as well as someone who will carry out small-scale work. 

Pick the professional who can demonstrate completion of similar schemes that impress you. In addition, you may want to choose a firm that is located fairly near to your house (while some designers charge for an initial visit, others do this for free).

7. Finding your builder

North London House Extension by Caseyfierro Architects Сучасний
Caseyfierro Architects

North London House Extension

Caseyfierro Architects

You need to decide if you want to use a larger and faster (but more expensive) firm that may be more flexible, or a smaller firm that will be less expensive, yet could take a bit longer in completing the project. 

Whichever you opt for, make sure that any contractors you use have warranties in place. And always draw up a watertight contract to protect both parties.

Good luck! 

Seeking some visual inspiration? Then we recommend: A truly unique home extension.

by Casas inHAUS Сучасний

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