Home extensions: what you need to know

Published by Hannah Nemeth on 11 March 2013.
Last updated on 10 February 2017

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

The cost of moving home went up, on average, by £870 (9%) between June 2015 and June 2016 to £10,996, according to research by Lloyds Bank. This was put down to higher property prices, which in turn pushed up the cost of estate agents’ fees and stamp duty – and it is the latter that can really deter people from moving home.

For example, using online property portal Rightmove’s Moving Cost Calculator, buying an average priced house in St Albans for £534,272 (without selling a property) would cost £16,714 in stamp duty, £2,671 in legal fees and £2,671 in other costs – a total of £22,056.

But if, instead, you put a little more money towards improving your home, you could 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. This would add £106,854 to the value of the St Albans house.

How much will it cost?

As a rough guide, the Royal Institution of Chartered Surveyors (RICS) suggests a homeowner would pay £1,480 a square metre for a 20 square metre loft conversion, which works out at £29,600. This is for a plaster finished shell with dormer and Velux windows and includes construction costs but not architect’s fees, which can cost around 15%. RICS says a single-storey extension

with a flat roof will cost £2,360 per square metre for a nine square metre extension, with a total cost of £21,240 – before you put down flooring or install a kitchen. As you can see from the table that the larger the project, the cheaper it will be per square metre.

Planning permission

An extension to your house is considered a permitted development and an application for planning permission won’t be needed provided certain conditions are met. However, if you are thinking of extending, then there is an argument for doing it sooner rather than later.

More generous size limits for singlestorey rear extensions are currently in place, but work must be completed by 30 May 2019. You will need to take into account any extensions or loft conversions that previous owners have built when working out how much space you can add. If you plan to extend a new-build house, you may need to get permission from the developer, which may charge a fee for this.

The main rules for extensions to houses

  • No more than half the area of land around the original house will be covered by additions or other buildings.
  • No extension will project forward of the principal elevation or side elevation fronting a highway (which means that the new extension should not go beyond the front or side of the original house that leads on to the street). 
  • No extension is to be higher than the highest part of the roof.
  • Single-storey rear extensions must not extend beyond the rear wall of the original house by more than 3m if an attached house or by 4m if a detached house. However, the limit is increased to 6m if an attached house and 8m if a detached house until 30 May 2019. These increased limits are subject to providing the local authority with prior notification of the work. A neighbour consultation scheme will take place and if objections are received, the proposal might not be allowed.
  • Maximum height of a single-storey rear extension of 4m. 
  • Extensions of more than one storey must not extend beyond the rear wall of the original house by more than 3m.
  • Side extensions to be single storey with a maximum height of 4m and width of no more than half that of the original house.
  • Two-storey extensions to be no closer than 7m to rear boundary.
  • Roof pitch of extensions higher than one storey to match existing house.
  • Materials to be similar in appearance to the existing house.
  • No verandas, balconies or raised platforms.


Loft conversion rules

Planning permission is not normally required for loft conversions, although there are limits to the extra space you can create, which are:

  • A volume allowance of 40 cubic metres additional roof space for terraced houses or 50 cubic metres for detached and semi-detached houses.
  • No extension beyond the plane of the existing roof slope of the principal elevation that fronts the highway.
  • No extension to be higher than the highest part of the roof.
  • Materials to be similar in appearance to the existing house.
  • No verandas, balconies or raised platforms.
  • Side-facing windows to be obscureglazed; any opening to be 1.7m above the floor.
  • Roof extensions, apart from hip to gable ones [a typical roof with two sloping sides], to be set back at least 20cm from the original eaves.
  • The roof enlargement cannot overhang the outer face of the wall of the original house.

For a full guide to the rules, visit Planningportal.gov.uk.

Extending a flat

The planning rules for flats and maisonettes are stricter than for houses. To add an extension to your flat, you will have to apply for planning permission. Similarly, if you’re planning to convert the loft in a top-floor flat, the rules are a bit woolly and you will need to check with your local planning authority.

You may not need it if alterations are just internal, but it will be required if you want to extend or alter the roof space. You will also need to check whether you own the roof space. If you are a leaseholder, the freeholder is likely to charge a fee for this.

Building regulations

You will also 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. If work will be carried out on a shared wall, you will need their co-operation to sign a party wall agreement before you start.

This will involve paying for surveys of the wall to take place both before and after the building work. The Construction (Design and Management) Regulations 2015 set out what those involved in construction work need to do to protect themselves from harm.

Builders working on any building project – including residential property – have to produce a construction phase safety plan.

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

Hiring a builder

The collapse of a Victorian terraced house in Lewisham, south-east London, last June – after internal load-bearing walls were apparently removed – 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 if you can’t find a builder through word of mouth, then try searching for a tradesperson on the Trustmark website. TrustMark is a government-endorsed scheme listing tradespeople who have been thoroughly vetted.

Book a verified tradesperson through home services marketplace Plentific.com, and you will get an insurance-backed guarantee that includes payment protection, work-in progress cover in case the tradesperson disappears half way through the job, post-completion cover – for any snagging jobs, for instance – and legal protection up to £50,000.

The National Federation of Builders offers a find-a-builder facility on its website – visit

Builders.org.uk. Similarly, on the Federation of Master Builders’ (FMB) website (Fob.org.uk), post your project details and your postcode and up to five FMB members will contact you. 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. Also check whether the builder has up-to-date personal and public liability insurance and ask to see their insurance certificates to verify this.

Ideally, you should have a written contract with your builder covering as much detail 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.

However, 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 builder to pay VAT – but remember you’ll have no comeback if the work doesn’t come up to scratch.

Brian Berry, chief executive of the FMB, says: “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.”

Don’t forget insurance

If you do have a party wall agreement, consider taking out Non-Negligence (party wall) insurance. This will protect you against any structural damage that comes to light either at your property or your neighbours’ in the months after the project has been completed – at a time when it is hard to prove that the problem was the builder’s. Prices start at around £1,100 a year for this type of policy.

Homeowners should not assume that their building project would be covered by their existing home insurance, as this is rarely the case. Contact your insurer to check whether it will restrict your cover. If so, you may need to shop around for a new policy or consider taking out specific insurance for this type of work.

Andrew Boldt, managing director of specialist insurance broker Insurance Tailors, says: “Make sure you tell your home insurer you are doing works as soon as you decide to renovate. Most insurers will at minimum reduce your cover during renovations – typically only covering you against fire, lightning, earthquake, aircraft or explosion – but in many cases will remove cover completely, putting you in a potentially awkward situation. It is highly likely that for structural property works over £100,000 in value you will need to take out specific renovation insurance.”

"I designed our extension on my daughter’s blackboard"

When Stephen and Bea Games moved into their Edwardian ground-floor flat in north London in 2013, one of the first things they planned to do was refurbish the rear extension, a cheap tack-on built in 1981.

Its windows were in poor condition and, worse still, the edge of its flat roof joined on to the French windows in the couple’s bedroom, which looked ugly and blocked out natural light.

They decided to divide the space into two separate studies—one for each of them—with a new V-roof and three Velux windows to let in more light. The French windows would go, and a half wall and stone-paved steps would lead up into the bedroom.

“The original extension was rotten physically and conceptually,” says Stephen, a designer and architectural historian. Stephen set about planning the new extension, but faced a couple of obstacles that delayed progress by five months.

“Flats don’t have permitted development rights even when you own a share of the freehold, and buildings in conservation areas require planning permission even when you’re not changing their footprint, just re-roofing and re-walling them,” he explains

Stephen did the design work himself, but hired an engineer to check that the concrete floor was strong enough to support a new brick feature wall to add character to the rooms. “Some aspects of the job were unusually timeconsuming,” says Stephen.

“Dumitru Durleci, our wonderful Romanian builder, had to be taught, for example, how to build brick niches and segmental arch heads for the internal wall, but he proved well up to the challenge.

“I explained everything face to face rather than with expensive architectural drawings. Where illustrations were needed, I sketched them in a notepad, in chalk on my daughter’s blackboard and even on the walls of the building itself.

“This was a more old-fashioned way of working, where the builder became a family friend and details were worked out on the back of an envelope,” Stephen adds.

It cost less than £20,000 to convert the original 5m x 3.6m extension into two smaller rooms and, once planning permission had been granted, the project took six weeks to complete.

For help with planning and design, you can contact Stephen at stephen.games@newpremises.org

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