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How Much Does an Extension Cost?

With the rising costs of moving house showing no signs of slowing down, it’s understandable why many people choose to have an extension built on their existing home rather than buy another. However, you also have to understand that many people have got the rest of their house and garden just how they want it. Furthermore, the entire family knows the area and the people within it. Compared to moving home, a house extension costs far less and is much less stressful.

So, the cost of an extension varies depending on the size and quality of the build. 

For a standard quality extension, the cost of build calculates to about £1,000/m2 to £1,400/m2

  • Small extensions. About 15m2, between £16,000 and £21,000 in total.
  • Medium extensions. About 24m2, between £26,000 and £34,000 in total. 
  • Large extensions. About 48m2, between £52,000 and £67,000 in total.

However, If you want a basic quality extension, the cost to build calculates to about £800/m2  to £1,000/m2.

  • Small extensions. About 15m2, between £12,000 and £16,000 in total.
  • Medium extensions. About 24m2, between £19,000 and £26,000 in total. 
  • Large extensions. About 48m2, between £38,000 and £52,000 in total.

Or, if you want a premium quality extension, the cost to build calculates to about £1,400/m2  to £2,000/m2.

  • Small extensions. About 15m2, between £21,000 and £30,000 in total.
  • Medium extensions. About 24m2, between £34,000 and £48,000 in total. 
  • Large extensions. About 48m2, between £67,000 and £100,000 in total.

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Remember, the amount you pay will vary depending on where you are in the country. Therefore, if you live in the South-East or London area, you’ll pay at least £20,000 extra.

A kitchen or bathroom extension which is 5m x 5m will set you back between £5,000 to £10,000+ as a rough estimate.

Additionally, professional fees, architect, structural engineer, planning application etc. costs about 15% extra on top.

If you chose a double-storey extension, the cost of materials will probably be about 1.75 times the cost of singlestorey materials but the labour will probably be about double. 

All prices are approximate and given without VAT.

When building an extension, you must always consider the cost and what you actually get for your money. Usually, as with most things in the building trade, the cost of a single-storey extension will vary depending on the size of the proposed use.  

A small single-storey extension often will cost almost as much as a larger one, so when we talk about potential quotations, we need to have it clear in our heads to choose the largest size extension that our money and the planning department will allow. The architect chosen to draw up the plans will be aware of all the planning rules and regulations in your area and will advise you accordingly. 

Bear in mind that these are indicative prices only and will vary depending on many different factors. It is far better to approach 3 or 4 different building companies and ask for a quotation.  A generic quotation will probably be free of charge but a detailed one will probably cost up to a couple of thousand pounds which will usually be refunded if you choose to go ahead with the building company.

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Types of Extensions 

The types of extensions you can choose are listed below but the most common is a side house extension. This is a general extension built on the side of the house, as opposed to being situated on top of an existing single storey. Furthermore, the main problem with building onto an existing single storey is that the foundations will need strengthening to take into account the extra weight of an additional storey. 

Single Storey House Extension 

A singlestorey build is probably a lot easier and cheaper than you think. Before you start you need to consult an architect to draw up the plans. You will need this to submit plans to the local planning department to see if you need planning permission. Whatever happens with their decision, you will definitely need building regulations which deal more with how the quality of the work and materials of the proposed extension complies with the relevant health, safety and environmental regulations.  

After the plans have been passed, they will be needed by the building company to calculate a quotation for the cost of the work. Once building starts, the various trades will each need a copy of the plans so they know exactly what you want and what has been agreed with the local council. 

Double Storey House Extension (TwoStorey) 

Building a twostorey extension will be very similar to building a single storey. Apart from the costs, which we’ve already talked about in the previous section, the main differences will be:  

  • Youll probably have to build the same kind of roof as the main house rather than a flat roof. 
  • The foundations must be able to support the additional weight.  
  • Upstairs windows must be suitable for a fire escape. 

Building a twostorey extension has one main advantage that outweighs all others. You get double the amount of floor space on the same ground area.  

Extension with kitchen or bathroom 

A kitchen or bathroom extension is always a good use of an extension because each of those is a room that isn’t easy to convert from an existing room. Remember that the plans will need to account for the plumbing of hot and cold water to the room as well as some way of removing the wastewater 

It doesn’t matter whether you have mains drainage or private drainage, the plans must show how you connect the extension to either the existing drains or some new drains. Mains water and drainage systems have very strict requirements when it comes to building regulations and planning permission and when dealing with domestic drinking water, you must comply with the UK Water Regulations. Therefore, always leave plumbing and drainage to the professional.

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Bungalow Extension 

Because a bungalow is basically a single storey house, the costs are similar, but the extension is built with a roof to match the bungalow’s existing roof. 

Fittings & Furnishing

Extra finishes and fittings can add a few thousand to the total bill if you don’t consider them at the planning stage. Typical extras include: 

  • Highquality bathroom and kitchen fixtures. 
  • Although double glazed windows come as standard, there are a variety of frame materials ranging from softwood, hardwood, uPVC, and aluminium. You may also choose triple glazing too. 
  • Many people don’t give any thought to finishing off the site afterwards. You can either have the ground landscaped and planted professionally or use it as a DIY project. 
  • Type of heating. You can choose from a range of different heating options depending on your preference and budget. Ground –source, air-source, gas oil or electric central heating, and solar. 

You’ll require many trades to build an extension, just like building a house. Here are the usual steps in building an extension and the average day rate you’re likely to pay for each trade. 

  • Building foundations and brickwork need a general builder or bricklayer. Day rate is from £150 to £240 /day. 
  • Carpenter for structural (including roof timbers) and finished woodwork, earns about £130 to £170 /day. 
  • Plasterer for plastering wall and ceilings. Sometimes needed for rendering the outside and earns about £130 to £170/day. 
  • A plumber installs all domestic water fittings and heating. He earns about £150 to £270/day. 
  • An electrician installs all the electrical fittings including lighting, heating controls and wall power sockets. He will earn about £150 to £280/day 
  • Roofer installs leadwork waterproofing and roof tiles. Average day rate is £150 to £220/day. 
  • Double glazing installer will fit external doors and windows and earns about £120 to £180/day. 
  • The painter and decorator is usually the last trade in a house and earns £120 to £180/day. 

Remember that prices are exclusive of VAT and doesn’t include labourers who will earn between £75 to £110/day. 

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How Long Does a House Extension Take to Build? 

A typical house extension will probably take about 14 to 15 weeks depending on size, quality and any delays. 

Let’s have a look at this: 

Week 1. Site Preparation. 

  • Prepare the site for building. Fence off the area so children and animals can’t get in the way. 
  • Create a storage place for materials and ensure it’s secure. 
  • Create an area for waste handling and the waste skip. 
  • Arrange the first delivery. 

Week 2. Groundwork & Foundations. 

  • Mark out trenches and excavate. 
  • Install services and drainage. 
  • Arrange for Building Regulations approval of trenches. 
  • Pour concrete foundations. 
  • Building Control approves foundations. 

Week 3 to 4. Groundwork and Lowlevel Work. 

  • Dig drains. 
  • Bricklayers to construct wall up to damp course. 
  • Install damp course, insulation and lintels. 
  • Pour concrete floor. Allow time for setting. 

Week 5 to 6. External and Internal walls. 

  • Lay bricks. 
  • Insert insulation. 
  • Install lintels. 
  • Fit windows and external doors frames. 
  • Lay internal blocks. 

Week 7. Build the roof structure. 

  • Erect roof rafters and ridge. 
  • Fit fascia and bargeboards. 

Week 8. Fit roof Covering. 

  • Fit roofing felt. 
  • Nail roof battens in place. 
  • Lay roof tiles and ridge tiles. 
  • Lay floor screed and leave to dry for a couple of days. 

Week 9 to 10. Guttering, Doors, Windows Render. 

  • Install windows and doors to frames. 
  • Render external walls if necessary. 
  • Install guttering and rainwater goods. 
  • Inside first fix carpentry. 
  • Install plumbing and electrical wiring first fix. 

Week 11 to 12. Knock through and plastering. 

  • Knock through into existing house to create an opening. 
  • Building control to inspect lintels. 
  • Box in as needed. 
  • Fit plasterboard & plaster. 

Week 13. Second Fix. 

  • Electrical. Lights, switches etc. 
  • Plumbing. Taps, connectors etc. 
  • Carpentry. internal doors, skirting etc. 
  • Painting & decoration. 
  • Carpets. 

Week 14. Snagging and clearing up. 

  • Minor issues depending on trades. 
  • Clean up. 
  • Finish garden.

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House Building Planning Permissions & Building Regulations

Extensions must comply with the various regulations just like a full-size house. 

However, if you’re in any doubt as to which regulations you have to comply with, consult your local authority or your architect. 

Professional Services Fees

Not only do you require people to build your extension, but you also need to employ additional professionals to ensure you’re complying with the various regulations. 

  • Architect. Draws up plans and often liaises with the local authority. 
  • Surveyor. Sets out the building and provides levels and heights for the builders to work to.
  • Structural Engineer. Calculates size and specifications of structural components such as foundations, supporting walls, roof timbers etc 
  • Local Authority Fees. Fees for submitting plans and for Building Control checks.  
  • Project Manager. Organises trades, schedule and orders materials. He can also liaise with the local authority. 

You should check with each one to find out what they charge, as their fees will vary depending on their services. However, usually, the total amount of professional fees range from 5% to 15% of the total job cost. 

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House Extension FAQ 

How far can you extend your house without planning permission? 

Usually, this depends on whether your extension comes under the definition of a ‘permitted development’. However, generally, the following restrictions are in place. 

  • No extension can exceed more than half the original area of the building. 
  • Extensions can’t be higher than the original roof. 
  • You can’t build extensions forward of the main elevation. 
  • Work cannot be a verandah or balcony. 

Single storey extensions can’t extend more than 4m from the rear of the house if it is detached. Otherwise, the limit is 3m. 

Double-storey extensions can’t extend by more than 3m from the house or be within 7m of any boundary. 

Remember, if you’re in doubt, consult your Planning Authority.  

Is it cheaper to do a loft conversion or an extension? 

Generally, a loft conversion is cheaper than an extension.  Mainly, this is due to the loft work usually taking place within the existing building. Understand, there is already a structure to work with as well as a roof. An extension, however, needs new foundations, walls, roof and so on.  

Furthermore, because the loft conversion takes place within the existing boundaries and footprint, you are constrained. Whereas, an extension can move outside and extends the building’s footprint.

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1 thought on “How Much Does an Extension Cost?”

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