Home extensions: what you need to know

When you've outgrown your living space, one possibility is to move house but when you weigh up the cost of upsizing, it's easy to see why so many homeowners decide to stay put and extend their homes.

Using Rightmove's Moving Cost Calculator, buying an average priced house in St Albans for £494,777 would cost £14,739 in stamp duty, £2,474 in legal fees, and £2,474 in other costs – a total of £19,687.

But if you put a similar sum towards improving your current home, you would enjoy the added benefit of your property increasing in value. Adding a bedroom and a bathroom through an extension or loft conversion can increase a property's value by more than 20%, according to Nationwide - which would make the value of the St Albans home just under £594,000.


How much will it cost?

As a rough guide, the Royal Institution of Chartered Surveyors (RICS) suggests that the cost of a loft conversion for a plaster-finished shell, excluding decoration and floor finishes, is £660 to £900 per square metre including VAT. London-based builders Simply Loft estimates that an average-sized loft measures 35 square metres, so, according to RICS price guidelines, it would cost between £23,100 and £31,500.

Extensions are more expensive: RICS gives a guide cost of between £1,260 and £1,680 per square metre for an extension with plastered finish. So a 20-square metre single-storey extension will range from £25,200 to £33,600 – before you've put down flooring or installed a kitchen.

Remember that if you are planning to extend a new-build house, there may be a clause in the original contract saying you need to get permission from the developer, which may charge a fee for this.

Planning permission

An extension to your house is considered a permitted development and planning permission isn't needed provided certain conditions are met. The main rules are:

  • Materials used in the exterior work should be similar in appearance to those of the exterior of the existing house, which is also the case with loft conversions.
  • The width of a side extension must not be greater than half the width of the original house.
  • Side extensions must be single storey with a maximum height of 4m.
  • If the extension is within 2m of a boundary, the maximum eaves height should be no higher than 3m.
  • A single-storey extension must not extend beyond the rear of the house by more than 3m for an attached house or 4m if the house is detached.


Until 30 May 2016, householders can build larger single-storey extensions under permitted development. The size limits have doubled from 4m to 8m for detached houses and from 3m to 6m for all other houses, though householders will need to provide their local authority with a written description of the work and a plan of the site.

Loft conversions are also considered permitted developments and the main rules (for England) are:

  • Any additional roof space created must not exceed 40 cubic metres for terraced houses and 50 cubic metres for detached and semi-detached houses.
  • An extension beyond the plane of the existing roof slope of the principal elevation that fronts a highway is not permitted development.
  • No part of the extension can be higher than the highest part of the existing roof.
  • Verandas, balconies and raised platforms are not permitted development.


For a full guide to the rules, visit the government's website at planningportal.gov.uk.

Building regulations

You will need Building Regulations approval for both extensions and loft conversions - for example, to ensure that the new floor and existing roof is structurally sound in a loft conversion or to check the foundations, roof, flooring, drainage and electrics in an extension. First, you'll need to get plans drawn up by an architect and approved by your local council's building control team before starting work. You will also need to see if the work you are planning falls within The Party Wall Act 1996.

On 6 April 2015, the Construction (Design and Management) Regulations 2015 came into force, which sets out what those involved in construction work need to do to protect themselves from harm. Builders working on any building project – including residential
property – will now have to produce a construction phase safety plan.

Where there has been more than one contractor on a project, homeowners will now have to present a health and safety file at the end of the job. This will include drawings and specifications that reflect how the project was built and it is likely that solicitors will request this file when a property is sold.

Contact Building Control at your local council for more information and, again, check out planningportal.gov.uk.

Choosing a builder

The collapse of a family home in Finchley, north London, after a botched basement dig, which recently hit the headlines, just goes to show how careful you have to be when hiring a builder. A good starting point is to ask friends and family for recommendations – but you also need to see how well you get on with the builder when you first meet.

On large-scale building projects, you could be liaising with the builder and his team for several months, so if you don't feel you have a rapport with them, then it's wise to choose someone else.

If you can't find a builder through word of mouth, the National Federation of Builders offers a find-a-builder facility on its website (builders.org.uk). Similarly, on the Federation of Master Builders' (FMB) website (fob.org.uk), you can post your project details and your postcode, and up to five FMB members will contact you.

But hiring a member of a trade body doesn't mean you can relax and hire the first firm that gets in touch. Don't be shy about asking builders for references - and follow them up with a phone call or even ask if you can visit completed projects. Once you have a shortlist, get a written quote from three firms. Also check whether the builder has personal and public liability insurance and ask to see their insurance certificates.

Ideally, you should have a written contract with your builder covering as much details as possible, such as start and completion dates, the cost of labour and materials, working hours and a payment plan that outlines at what stages payments are to be made.

Brian Berry, chief executive of the FMB, says: "BACS is typical these days but on smaller contracts, cheques are still widely used. A request for a deposit, particularly to cover the cost of drawings, is quite standard but householders should be wary of any builders demanding a high proportion of the final cost upfront."

While it's better to have a written contract, it's not unusual for homeowners to have a verbal agreement with a builder, with cash paid every couple of weeks. It is likely to make the job cheaper – as the onus is on the self- employed builder to pay VAT – but remember you'll have no comeback if the work doesn't come up to scratch.

"The cost can really mount up"

Our extension has taken the best part of a year to complete, from us first deciding we wanted to build one to it finally being finished. We had to find an architect and a builder, as well as research whether we needed planning permission, which involved liaising with the local authority. All of this takes time.

There were architectural plans to be drawn, council planning officers needed to inspect our property, surveyors needed to visit the property and – as we are semi-detached – draft a party wall agreement.

We spent hours researching reputable builders on Checkatrade.com and took recommendations from friends, before asking four firms to quote – cost was key for us. Eventually, we found a fantastic builder - H&C Construction Sussex - which completed our 18-square metre build within about 10 weeks for £27,000 including VAT.

Of course, there were hiccups. We discovered our boiler needed to be moved from the kitchen to the loft, which set us back a week or two, and the weather proved problematic at times but the process went smoothly enough.

My main advice would be: the build is one thing but you must also factor in added costs such as flooring, furniture and anything better than standard fittings. The cost can really mount up.

Mark King, Moneywise editor