How to plan and design an extension

Forget moving - make the most of what you have by extending your house up, out or down...

Why move when you can improve? There are lots of reasons to extend, whether you want to create more space, add value to your home or just love the area in which you live. ‘The increasing cost and hassle of moving combined with uncertainty in the property market makes staying put an attractive prospect,’ says Hugo Tugman, co-founder of Architect Your Home. ‘Many properties have untapped potential, and renovations can make a home more enjoyable to live in as well as adding to its market value.’

Things to consider before you start:


Before you plan that extravagant extension or costly conversion, it’s crucial to talk to a local estate agent to find out what renovations are popular in your area. Every street has a ceiling value so don’t over develop if you want to see a good return on your investment.


‘If your area is full of young professionals, family-friendly extensions are not the way to go,’ says Michael O’Flynn of ‘Ask agents for examples of extended properties they’ve sold and how much they went for.’ has access to the Land Registry’s sold prices info, plus properties currently on the market. ‘Plan wisely and you’ll add more value than the amount spent,’ adds Tugman.

Extension trends

Modern garden terrace | Modern extensions | Extension ideas | PHOTO GALLERY |

While most contemporary extensions are a world away from those awkward structures that attempt unsuccessfully to blend in with an existing building, they’ve moved on from the ultra-modern glass boxes and mixed materials of the past few years. ‘People are still happy to change the structure of a building and do something a bit more dramatic, but purity is a big trend, and using lots of different materials is less fashionable than it has been,’ says architect Guy Morgan-Harris. ‘Instead, opt for something more refined. Pure white-rendered extensions work well on period houses, or choose a design clad entirely in timber.’

Judith Tugman of Architect Your Home agrees: ‘Clients want simple, light, airy spaces – a blank canvas they can decorate and personalise. We’re using natural materials to create modern extensions that are warm and functional.’

With more people working from home, studies are becoming an increasingly popular reason to extend. ‘We have lots of clients who want a work space that can be hidden away at the fold of a
lever,’ says Guy. ‘Many people are also converting rooms at the front of the house into an office, and redesigning the back of the house as the main living space. As a result, having your audio-visual equipment,plasma screen, lighting and Wi-Fi integrated into the structure is aclever idea.’

Unsurprisingly, there’s also been a big move towards sourcing more environmentally sound materials, and government regulations mean that any new structure must adhere to energy-saving guidelines. ‘Materials such as bamboo, which is affordable, durable and eco-friendly, are very popular, and people are also requesting low-VOCpaints,’ says Guy. ‘One in four lights in new structures should be low energy, and proper insulation is also key. In the long run, of course, all of these elements will cut down your energy bills as well.’

Why employ an architect?

White open-plan kitchen extension | Modern extensions | Extension ideas | PHOTO GALLERY |

‘It sounds obvious, but builders specialise in building things and architects specialise in designing spaces,’ explains Judith Tugman ofArchitect Your Home. ‘Going straight to a builder means you miss out on the crucial design stage. By using an architect, you’ll end up with a
home that’s right for you and that you’ll love living in. Remember, too a badly designed extension can actually reduce the selling price.’

You can hire an architect to manage all of the building process, including helping you secure planning consent and building regulation certification, finding a suitable builder, monitoring progress, standards and safety on site, arranging input from specialists and overseeing construction to completion.

‘Your relationship with your architect is very important, so it’s crucial to find someone you’re confident you can work with,’ says Mark Dyson of Enclosure Architects. One of the best ways is through personal recommendation, or contact RIBA, which can provide a shortlist of architects with the skills to suit your project. Log on to for a directory of registered practices.

When you find an architect who appeals, ask to see their portfolio. Look at three recent jobs and visit finished projects. Don’t forget to ask for references, and bear in mind a good practice will be in demand.

Preparing a brief

Supersize an island to make a big impression

‘Design is not the same as art,’ says Mark Dyson of Enclosure Architects. ‘An architect is, in fact, a problem solver, not an artist. Consider what you want from your extension and how it will make your life better.’ Sit down with your architect and identify what you want to
achieve. Don’t overdo the brief, but make sure it’s unambiguous and well-defined.

  • Consider how the finished project will function. Do you need an extra bedroom, crave a bigger kitchen or want more light in your home? Who will use it, and for what?
  • Think about the design direction. ‘Have you already seen something you like the look of?’ asks architect Guy Morgan-Harris. ‘Do you want it to contrast or be in keeping with the
    existing structure? It’s really handy to collect images of projects that appeal to you.’
  • Consider internal finishes such as flooring, lighting, storage and even smaller things like light switches in the early stages, as they’ll all affect your budget.
  • Agree on a timescale. ‘If you rush a job you may lose outon quality, and the project may end up costing more as there will be less time to source materials,’ says Guy.

Setting your budget

‘Clients are sometimes cagey about money,’ says Judith Tugman, ‘but if you give your architect a budget, they can assess what’s possible.’And discuss it as a team – your architect may even be able to do a job for less, in which case you could have more to spend on fittings.

  • A general rule of thumb is to allow £150 per sq ft of development, and remember you aren’t just thinking about the extension, but the existing space inside, too.
  • If you opt for less expensivefixtures and fittings, you could squeeze this down to £100, but equally, if you go for higher quality this could rise to £200 per sq ft.
  • Bear in mind that, on top of the build budget, there will be architect, engineer and builders’ fees, government VAT and building control and planning costs, too.
  • Don’t forget to allow a contingency fund of 10 per cent for unforeseen circumstances.

Ground-level extensions

Up until quite recently, the front of the house was the most social area of the home and the kitchen and rear were purely functional spaces. Now, with cooking the nation’s favourite pastime, open-plan kitchen-diners are increasingly in demand. ‘Buyers like spaces that
suit their lifestyle and aspirations, and side extensions and large living/dining areas appeal to the way people live now,’ says Michael O’Flynn. ‘Having a large, well-lit family space that has access to the garden is a big selling point.’

The latest ground-level extensions are seamlessly integrated inside, but also blur the boundaries between inside and out. ‘The flow of the structure, finishes and materials such as the flooring can work with the outside space to make them feel like the same entity,’ says Mark Dyson.
‘Full-height doors can help to connect the interior to the garden, and even in the winter when the doors are closed it can still give the impression it’s one large space.’

  • Providing your property hasn’t been extended in the past, you can extend by up to 50 cubic metres, as long as it’s not more than 4m above ground level, or 10 per cent of the existing floor space, without obtaining planning permission. Even so, always check before embarking on a project in case there are other conditions that apply.
  • Sometimes simply removing internal walls can open up a space without having to eat into valuable footage in the garden. And don’t forget that making use of a side return can increase the size of your kitchen by up to 40 per cent.
  • Part L Regulations, which relate to a building’s thermal efficiency, mean there is now a restriction on how much glass you can use, but this can be offset by installing high- efficiency insulation and by using Low E (low emission) double or triple glazed panels.

Loft conversions

Loft conversion with bathroom and dormer window

‘Loft conversions are always popular as they make use of existing space,’ says Michael O’Flynn. ‘Adding an extra bedroom and bathroom can boost the value of your property by 15-20 per cent, and take it into a completely new price bracket.’ Think carefully about who you employ, though. ‘If you choose an off-the-peg conversion company, you’ll get an off-the-peg solution,’ points out Judith Tugman.[/caption]

  • Properties built before around 1975 are most likely to be suitable. Modern trussed roofs may need more structural work, and an attic roof needs to be at least 2.3m high unless you’re prepared to have the ceiling below lowered.
  • To be considered a proper room, it will need a permanent staircase. If you’re planning a bathroom, think where soil pipes will go and whether your existing boiler could supply extra hot water.
  • Your local planning office can tell you if you need planning permission, as it can vary. You’ll also need to comply with building regulations, including fire safety requirements.

Basement conversions

Clean-lined modern fitted kitchen with table, chairs and wooden stairs.

Basement conversions are the most expensive and specialised projects toundertake, but a good idea if you live in a very high-cost area.‘Conversion companies will charge around £250-£300 per sq ft to create extra living space under your existing home, and a 20 x 20ft basement in
a London terrace can cost anything from £100,000 to £120,000 to convert,’ says Michael O’Flynn. ‘However, it can provide a whole new floor, and internal living space in the most sought-after areas can cost more than £1,000 per sq ft, so the overall gain can be well worth the

‘The majority of conversions we do are very much about leisure,’ says Maggie Smith of The London Basement Company. ‘We create big family rooms with built-in plasma screens and audio equipment. It’s important to remember to incorporate good storage, as well.’

  • Older period properties tend to be more suitable as they have sturdier foundations, and many already have cellars, meaning foundations are even deeper. More modern
    houses can sometimes be converted, but check with an underpinning specialist to assess feasibility and cost.
  • If your house is a terrace, you may need to underpin your neighbour’s property as well as your own, and will need building regulations approval to cover damp-proofing, ventilation and wiring. Planning permission isn’t usually necessary for cellar conversion, but
    is for a new basement.
  • If a house has a timber floor, work can begin from outside, meaning you can live there during work. If the floor is concrete, work must start by going straight through, so you’ll need to move out for a while. You can tank a basement, but if the water table is too high, the cost will be prohibitive.
  • How long a conversion takes depends on the project size, but an average job of 50-80 sq m takes around 20-24 weeks.

Useful contacts

3S Architects and Designers


Architect Your Home

Bauman Lyons Architects

BB Partnership

Dominic Stevens Architecture 00 353 719 636988

Eger Architects

Eldridge Smerin Architects


Found Associates


Ian Hay Architects

The London Basement Company

LP Architects

Martyn Clarke Architecture

Milk Studio

Morgan Harris Architects

Paul Archer Design

Plastik Architects

Point 3


William Tozer Architects

More top tips on extending your home

Have you recently extended your home? We would love to hear your thoughts and see you pictures.

Ideal Home loves...

Children's room with blackboard wardrobes | Country children's room design ideas | Children's room | PHOTO GALLERY | Style at Home |

8 super simple DIY hacks for your child’s bedroom

Dotty runner | Hallway runner ideas | PHOTO GALLERY | Country Homes and Interiors |

Why every home should have a hallway runner


Beautiful boot rooms for country pads

1.jpg | Country Homes & Interiors |

7 beautiful oak-framed extensions

Working from home? Do it in style…

Feminine hallway with butterfly motif | Unforgerttable hallways | Hallways | PHOTO GALLERY | Livingetc |

How to make an unforgettable first impression