Cost of Building an Extension in 2019

There are many benefits to building an extension to your home compared with moving house. One of the most stressful periods of your adult life is when you move, not to mention the tax payable with Stamp Duty charges. But, if you are happy with the area where you live, the best alternative is to build an extension. “Improve, don’t move”, is a very popular saying and one which has a lot of truth. Not only do you keep your friends and neighbours, but you also keep happy memories built up over the years. You can also improve on your already satisfactory accommodation with something designed exactly how you want it.

Extensions are an ideal way to increase your living space, and add value to your dwelling when it comes to selling later on.

This guide will show a breakdown of extension cost estimates, professional and associated fees, and the mandatory regulations required by law.

Quick Preparation Checklist

Before we leap into the specifics and deal with all the numbers, it’s a good idea to look at some considerations to make everything run smoother, before beginning your development work.

  • Insurance – protects all parties from financial risk.
  • Architect – draws the plans and submits them for planning permission.
  • Building Control – checks the work is of the proper quality. Contact your council.
  • Planning Permission – provides the license for the development. Contact your Planning Department.
  • Skip Hire – provides the means to handle and dispose of building waste.
  • How much do you want to pay? – Always have a rough idea of your budget.

Typical Extension Costs

Giving a quotation for a typical extension can be extremely complex because many individual operations must be accounted for.

Builders therefore generally quote per square metre (cost per m2) and lump many small individual costs into one.

Common Types of Extension

All estimates here are approximate, exclude VAT, and are rough indication only.

Single Storey Extension

This ground floor structure has its own foundations designed to carry its weight and area.

For a typical good quality 5m x 5m addition you will have to pay out about £30,000, with an extra premium of £20,000 if you are in London and the South East. A structure like this will take about 14 weeks excluding any holdups.

Double Storey Extension

This consists of a ground floor and first-floor structure. This still needs a roof and foundations so won’t cost much more than a single storey (but the roof is less accessible).

Typical cost is about £45,000 with an extra £25,000 in London.

Build time for a double storey addition will be about 6 months.

Comparison Table

If you want to have a more detailed breakdown, check out the table below. Remember all numbers are approximate and need confirmation by your builder. They are characteristic of high-quality building standards and are correct at time of writing.

The underlying construction work will cost approximately the same for whatever quality of extension you are building. It’s the quality of visible fixtures and fittings that determine the relative budget variations. For example, a standard internal door retails at £250 per door, a good quality door is £350 and a high-quality door comes in at about £450 each. Similarly, kitchen and bathroom fitments also significantly alter the outlay.

As a rough guide, the outgoings on a standard and good quality build compared to a high-quality one will be approximately 50% and 75% respectively.

Extension Type Description
Cost per m2 Estimated Duration
Single Storey Extension Ground floor structure £1,200 14 weeks
Double Storey Extension Ground floor & first floor. Usually, add 50% to single storey extensions. No closer than 7m from rear boundary £1,800 26 weeks
Lean-to Extension Same as single storey but has a simpler roof style £1,200 14 weeks
Rear Extension Depends on single or double storey. Maximum 8m extend for a single storey and 3m extend for double £1,200 or £1.800 Varies
Wrap Around Extension Because it has corners and angles it can make the roof very complicated and expensive £1,200 Varies
Side Extension Only single storey with maximum height of 4m allowed. Width less than half the building £1,200 14 weeks
Kitchen Extension Add plumbing and fittings costs to structure £10,000 total extra Varies
Bathroom Extension Add plumbing and fittings costs to structure £5,000 total extra Varies
Bedroom Extension Depends if it is part of a single or double storey structure Varies Varies
Bungalow Extension Similar to single storey extension £1,200 14 weeks

To calculate the final approximate cost, multiply the area of your extension (in m2) by the cost per m2. Add professional fees 10% to 15% (architect, surveyor and structural engineer) and Local authority inspection and licence fees (about £200). Finally, add VAT which at present is 20%.

Remember, each location has a ceiling price for the value of a property so don’t go overboard and pay out too much for a top quality home.

There are many estimation tools you can use, has a great extension cost calculator – keep in mind these approximate costs are exclusive of VAT.

Finishing & Fittings

When you’re researching your extension quote, the amount can vary considerably by things not particularly obvious and therefore not accounted for. It’s worth looking at these charges to see how much they are, so let’s look at those for a typical single-storey extension. Assume about double for a two-storey extension.

You are spending a large amount on your extension so it makes sense to get the best workmanship you can afford. Rule out those tradesmen who are not up to scratch by asking whether they have qualifications and belong to professional associations.

Type of Work Typical Dayrate Qualification / Accreditation
Electrical £150 to £280 Part P Registration, NICEIC ID, ELECSA ID, NAPIT ID
Plumbing & Heating £150 to £280 Gas Safe qualifications, City & Guilds, NVQ, Chartered Institute of Plumbing and Heating Engineering
Plastering £130 to £170 City & Guilds, NVQ
Double/Triple Glazed Window Installer £120 to £180 FENSA, CERTASS
Carpenter £130 to £170 City & Guilds, NVQ
Roofer £150 to £220 City & Guilds, NVQ
General Builder £150 to £240 City & Guilds, NVQ
Labourer £75 to £110 N/A

Day-rates depend on experience and location in the country. Additionally, all tradesmen need Public Liability Insurance to cover their work. Electrical and Heating professionals need additional insurance cover.

Hiring Extension Builders – Top 10 Tips

When looking for your extension specialist, ask the following questions and understand what you’re getting into.

  1. Get proof of accreditations & membership of professional organisations.
  2. Ask for the quote, including a breakdown of the work. Also whether there are specialist subcontractors and if it includes VAT.
  3. Is there a deposit and how much? Be wary of paying too much up-front.
  4. Agree on payment terms and work schedule.
  5. Ask for references and follow up on them.
  6. Ask how long they have been in business locally.
  7. Check the builder has Public Liability Insurance and ensure it’s valid for the required period.
  8. Ensure you have a written contract. Experienced professionals always provide one.
  9. Ensure the price of your extension is affordable. Only agree if you can pay.
  10. Be wary of dishonest traders and don’t be frightened of asking additional questions.

Professional Fees

Include professional fees into your budget. You will need to contact these separately for their services or via an architect. Remember, fees are without VAT.


  • The initial consultation is often free. But the usual fee for this (about £250) will be built into the final invoice.
  • The package submitted to the Planning Department includes a measured survey, preliminary designs, further development and application documents. This is usually about £1,500 to £2,000.
  • 3D Renders are a useful thing to have. They are a set of three-dimensional images or models to show the contractor, neighbours or other interested parties. Add on £500 to £1,000.
  • Architects prepare detailed designs for Building Regulation and often oversee surveyors and engineers. Expect this to be about £1,500.


Total so far is £3,750 to £4,750. On top of this, you have the cost to submit full plans (£180), inspection fees (£200 to £550) for Building Regulations as well as Planning Permission for House extension planning (£210) and full conversions (£460).


That isn’t all, you must also pay for a Flood Risks Assessment, an Ecology Report and a Tree report.

Structural Engineer

These fees are between £150 to £600 for small jobs, while larger jobs can be from 5% to 10% of the overall project cost.



  • For Party Wall Agreement they charge £150 to £200 per hour.
  • For Party Wall Award, charges are about £1,000.


Insurance Cover

Renovation Insurance is needed to cover your home during building work up to a value of £250,000. Your home may be less secure than normal without doors and windows in place and you will have strangers working in your home. The insurance company ensures the policy remains relevant during all stages of the building work.

If you are doing the project yourself you might need Self Build Insurance. Specifically, this is for projects where the main contractor doesn’t have a comprehensive policy such as employers’ liability, legal expenses and damage cover. Comparethemarket has an excellent insurance guide on the costs for a house extension.

Remember that if you are renovating or extending your home, your existing policies will be void when alterations are carried out. Notify your insurance provider in the planning stages and take their advice.

Building Project VAT Exemptions

There are certain circumstances when your builder doesn’t have to pass on VAT to you. Work such as building a new flat or house, and work for disabled people in their own home are both classed as zero-rated VAT.

VAT is charged at a reduced rate of 5% under certain circumstances.

  • Energy saving products and installation for homeowners over 60 years old.
  • Conversion of a building into a house or flats.
  • Renovating and modernising an empty residence.
  • Home improvements for any domestic property on the Isle of Man.

You can attain the full details on the government website here.

Planning & Legal Requirements

Local and national planning requirements, building control and other legislation surrounding work on residential extensions vary as circumstances and legislation change.

Here are some resources that go into further detail for each element:

  • The Planning Portal has an excellent guide on all aspects of planning and building regulations for domestic extensions. Compare your plans against the information supplied and see if they comply. If you need more help contact your local authority who will advise you accordingly.
  • All building work must comply with the Construction Design & Management Regulations 2015 (CDM). This information on the HSE website lays out the Health and Safety requirements of your project.
  • The Party Wall Act 1996  specifies responsibilities if your project stands on a boundary with a neighbour.
  • The Neighbour Consultation Scheme relates to how your project will affect your neighbours. Notify your planning department who then consult your neighbours. They add this evidence to your planning application.
  • If you aren’t sure which is your local council, enter your postcode into this search box to find out.
  • If your project affects established trees, find out if the tree is protected and comply with the legislation.
  • Check out this page on Planning Portal for information regarding foundations, drainage and associated topics.
  • Your building may be what is termed ‘Listed’ or you may be in a Conservation area. If so, discuss this with the Planning Department as you may have to comply with additional regulations.

* Changes from June 2019

Important news. Previously, temporarily introduced Permitted Development rights allowed certain work without planning permission. From June 2019, these rights are now permanent. This will help families grow without having to move house and will reduce bureaucracy.

Government Grants

Gone are the days when you could buy a derelict property and turn it into a home worth millions. The housing market is now unable to support that kind of speculation. However many local councils will help with the costs of renovating properties. They’re offered at the local council’s discretion so contact yours and find out if you qualify.

If you’re disabled, the chances of help are much easier (but definitely not guaranteed). Search this webpage and see what’s available in your area.

Local government grants have been severely reduced recently and not all councils can afford to offer them even if you seem to qualify.

Extension Cost F.A.Qs

How much value will an extension add to my house?

This varies depending on the quality, size and location of your home. Although this shouldn’t be the first thought when extending your house, the Office of National Statistics has a tool to help you value an extension.

How to finance an extension?

Financing will be tailored according to your situation – here are some pointers:

  • Remortgage your existing home with your existing or new lender. Check if you are in a fixed rate term on your existing loan as the fees might add up considerably.
  • Try to organise a secured loan. This is more like extending your existing mortgage.
  • Unsecured loans might work but you will pay higher interest rates.

The above should not constitute financial advice, these are our opinions and you should seek advice from a financial professional.

How long does an extension take to build?

Here is a table containing your typical extension build time in weeks:

Week No. Work done
Week 1 Prepare site and your neighbours for the work to begin
Week 2 Excavate, build foundations, Building Control inspection
Week 3 & 4 Dig trenches, lay drains, construct walls to damp proof course, pour concrete floor
Week 5 & 6 Build external and internal walls, fit window and door frames, install cavity wall insulation
Week 7 Construct roof frame, install dormers etc.
Week 8 Install roofing felt, battens, vents, lead valleys, fascias, soffits, roof tiles, lay floor screed
Week 9 & 10 Hang windows and doors, install guttering, first fix carpentry, electrical and plumbin
Week 11 & 12 knock through from house, install lintel etc. Building control, plasterboard, plaster
Week 13 Second fix electrics, plumbing and carpentry. Painting, wood flooring, carpets
Week 14 Snag minor problems, tidy the garden, tidy site, remove skips and tools

What are alternatives to an extension?

Converting the garage to a living space or bedroom can be a great option. The condition and location of the garage will determine what permissions you need and what you need to spend. This can increase the value of your home by up to 20%.

Next Steps

I hope that this guide on extension building costs gives you some insight into the possibilities of an extension for your home. It’s easy to find reliable builders in your area. Complete the form and receive up to three quotes for your project.

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