How to Add a Single Storey Extension

Building a single storey extension is a cost-effective alternative to moving house, allowing you to design additional living space with a layout that suits how you use your home. With expanses of glazing, it can also provide you with the opportunity to create a light-filled space that’s integrally linked with the garden.

This guide covers common extension planning questions such as:

  • Cost
  • Planning permission
  • Building Regulations
  • Party Wall Agreements
  • Size
  • Design

How Much Does it Cost to Build a Single Storey Extension?

Before you begin planning your extension’s design, you need to set your budget and ensure you can afford what you’re planning.

Start with build costs, which can be broken down as follows:

  • If you’re looking to build an extension on a budget, you can achieve a single storey extension for around £1,500 to £1,900/m² for basic quality (a 4m x 5m extension would work out at around £30,000)
  • For good quality, expect to pay between £1,900 to £2,200/m² (or £38,000 for a 4m x 5m addition)
  • For a high spec extension, you’ll pay between £2,200 to £2,400/m² (or upwards of £44,000 for an extension of 4m x 5m)

Use our Extension Cost Calculator for an idea of how much your extension will cost

On top of the build cost, you’ll also need to factor in the following:

  • Architects’ fees: These work out at around 3-7% of the construction cost, with planning drawings around £2,700, and construction drawings at a similar rate
  • Structural engineer: If roof joists and foundations need to be specified, you’ll need a structural engineer. This would cost in the region of £500 to £1,000 
  • Survey: Between £500 and £1,500 if a survey of the existing house is required
  • Project management: Factor in a fee of 3-7% of the build cost for project management (you can also agree a daily or hourly rate). If you are looking to keep costs down, you could always manage your own project
  • VAT: This is at a rate of 20% of the labour, materials and services
  • Planning fees: For a residential single storey extension in England, the cost of an application is £206. If you need a certificate of lawful development, you’ll pay £103; and it costs £34 per request for discharging planning conditions
  • Building control charges: These will vary according to your extension’s size; plan for between £200 (for an extension of 10m²) to £900 (for 80 to 100m²)
  • A party wall agreement: This typically costs from £700 to £1,000 per neighbour.
  • Additional fees: These can include a tree report (£250 upwards); a flood risk assessment within flood zones (£250 upwards); an ecology report, possibly required by your local authority(from £400); an archaeological report if your home is in an area of archaeological interest (possibly several thousand pounds); a historic building report, likely if your home is listed
  • Interior fit-out costs

A contemporary extension to a tired 1950s house has allowed the creation of a light-filled open plan kitchen-dining-living room that works well for the homeowner’s growing family

How Much Does it Cost to Fit Out a Single Storey Extension?

The costs involved in fitting out a single storey extension very much depends on the room type you’re adding — a kitchen will be more expensive to equip than a home office, for example — but expect to pay:

  • Between £5,000 and £30,000 for a new kitchen (higher spec kitchens can cost considerably more)
  • From around £4,500 to £11,000 for a bathroom, depending the quality of fittings; a shower room will cost a similar amount
  • Factor in between £25 to £100 per square metre for flooring
  • Add around £85 per square metre for plaster or dry-lining, plus paint if these finishes are not included in the build quote
  • Expect to spend between £1,500 to £2,000 per linear metre for bi-fold or sliding doors
  • Don’t forget to include the cost of adding heating to your new room. Extending an existing central heating system may only need two days’ work by a plumber, at around £150 per day (excluding materials). Underfloor heating will be more expensive. Electric underfloor heating is a cheaper installation choice (and some elements can be DIY-fitted). However, water-fed underfloor heating, although more expensive to install, and possibly requiring the addition of a new boiler to cope with the demand, is cheaper to run in the long term. Expect to pay around £2,500 for a new boiler

A contemporary extension to this 18th century cottage is the perfect mixture of sympathetic and contrasting design

Do I Need Planning Permission for a Single Storey Extension?

You will need planning permission for your extension if it:

  • covers more than half the area of land surrounding your home
  • extends towards a road
  • increases the overall height of the building
  • extends more than 6m from the rear of an attached house
  • extends more than 8m from the rear of a detached house
  • is taller than 4m
  • is more than half the width of your house
  • uses different materials to those of the original house
  • includes a balcony or raised veranda

A good local architect or builder will be familiar with these limitations, but it’s always worth double-checking yourself to avoid making a planning application more than once.

Can I Extend Within My Permitted Development Rights?

You may be able to build a single storey extension with permitted development (PD) rights, avoiding the need for planning permission, if its height and footprint meet permitted development criteria.

Always check with your local authority, especially if you live in a Conservation Area. Be mindful too that local authorities can also use Article 4 Directions to remove rights.

Even without planning permission, it is worth applying for a certificate of lawful development from your local authority; it could be valuable in future if you need to prove that your extension met PD requirements and did not need planning permission.

Do I Need to Comply with Building Regulations?

An extension of any kind must comply with building regulations. You will need to submit an application for the work to your local authority’s building control department. Each authority has its own table of charges or you can use a private certified building control firm. Some contractors are building control certified, meaning they can carry out work without the need for involving the building control department.

This striking extension is home to the family’s bedrooms in this post-war house in Hampshire

Do I Need a Party Wall Agreement? 

If you have a good relationship with your neighbours – or can establish one – and talk them through your plans, you may be able to persuade them to sign a party wall agreement waiver form.

Otherwise, you will need to appoint a surveyor to arrange a party wall agreements; your neighbours are entitled to hire their own surveyor if they don’t want to share yours, and this will be at your cost.

How Big Should My Extension Be?

Planning constraints, such as how much your house has already been extended, will limit your extension’s size, but bear in mind that the larger the extension, the more cost-effective.

That said, an extension that dramatically reduces the size of the garden can have a negative impact on the desirability of your home to future buyers. Also if you extend within permitted development criteria you can build without going through the planning process.

Extending for the first time? Here’s how to ensure project success

Designing a Single Storey Extension

Whether you have chosen an architect to work with or are working on the design of your extension yourself with an experienced builder, here’s what to consider.

Interior Layout

Open-plan, a series of smaller spaces or a combination of the two? Consider your needs carefully before you start. Building a kitchen extension might involve designing an open-plan kitchen diner and living space, but allowing space for a separate utility room and cloakroom, for example.

Doors and windows

The position, size and shape of doors and windows to capture the best of the daylight and the views of the garden can mean that even when building an extension on a tight budget, you can achieve a stunning space.

Ensure their style and framing complements your interior fittings – designing a kitchen to complement the style of your bi-fold or sliding doors, for example, will create an enhanced space.

Want to build an extension that breaks the mould? Be inspired by these standout projects

Creating a Seamless Space

Creating a natural flow from the original house into the new extension will help make it more successful. This may mean:

  • having a wider than usual doorway into the new room
  • hiding joists in the ceiling void to ensure a continuous ceiling level between existing and new spaces
  • making sure floor levels in the extension match those of the hallway it leads off from
  • and ensuring decorative elements, such as mouldings, door furniture and colour schemes, match those in the existing house

Bringing the Outside in

Merging indoor and outdoor spaces can make both spaces feel bigger and cohesive. Think wide bi-fold doors overlooking the garden, continuous flooring from the inside out and choosing harmonious features, like wall treatments to plants, to create a cohesive space that spans through the house into the garden.

A Modernist-style addition to an 18th century sawmill

Future-proofing

A single storey extension is cheaper to build than a two storey extension because it needs less substantial foundations and steelworks. But might you want to add a second storey in the future? If there’s the slightest chance, now’s the time to up the spec of the extension to allow for this in future.

The roof

Obviously, the loftier the ceiling in the new extension, the brighter and bigger the space you’ll create, but consider the impact of the height of the ridge on existing first floor windows. If a low pitch is the only option, you may have to get creative with your roof design, opting for an orangery-style roof with glazed elements that make the ceilings look higher than they would if solid.

Need inspiration for your extension’s roof? Check out these great design ideas

Exterior materials

Choosing these to match those of the original house can make your extension look like it’s always been there; however, choosing contrasting but complementary materials can work just as well, and is sometimes more acceptable to planning departments.

Lucy Searle is Editor-in-Chief at RealHomes.com

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