The time of poky kitchens and separate formal dining rooms is over. A combined kitchen-diner commands sociable behaviour, just follow these simple steps to create the heart of your home.

The dining room is out, and the kitchen is in. But not your scrubbing, chopping and boiling kind. This is the age of the kitchen-diner: an open family space where friends hover, teenagers graze, children do homework and into which appliance manufacturers beam music, TV and the internet. A space which grand designers across the country are busily widening doorways, knocking through walls, adding extensions and even burrowing into basements to accommodate. The kitchen-diner is to the modern house what the great hall was to the medieval castle and the parlour to the Victorian terrace. It is the heart of the home.

Space

Most kitchen-diners are at the back of the house, opening out on to the garden. This makes perfect sense: a kitchen-diner is nothing if not an exercise in lifestyle, and what could be more pleasant than flinging open the back door and bringing the outside in? In many homes, that back door is fully glazed, being one side of the streamlined glass box that is the popular kitchen-diner extension. But remember, under the latest building regulations new glazing panels must comprise less than 22.5 per cent of a property's floor area.

If you live in a flat, a pitched glass roof is off the menu anyway, but you can cook up a kitchen-diner by knocking through walls and turning two or even three small rooms into one large one. In Islington, London, kitchen designer Mark Evans, of Mark Nicholas Design, says this kind of remodelling constitutes much of his workload. 'A clever kitchen-diner typically reduces hall and corridor space,' he says. Increasing numbers of kitchen-diners are also being quarried on the underside of urban properties. Evans was recently involved in a project which saw the creation of two entirely new subterranean floors with a combined height of seven metres. What's more, the client could have carried on digging: you own the land on which your property sits, all the way to the centre of the Earth.

Planning Permission

Get your calculator out: you are allowed to extend up to 15 per cent of your property without planning permission (different rules apply in conservation areas). The extension must not be higher than the highest point of your house, but it can be lower: basement extensions don't require planning permission, just party wall consent. And, by the way, deep pockets: going underground with your kitchen-diner is an engineering feat that can cost thousands of pounds.

Flat extensions, meanwhile, always require permission. If you want to go bigger, but not down under, you're most likely to get permission to build at the rear of your property, but it very much depends on your local planning department and what precedent has been set by your neighbours. It hardly needs stating that it makes sense to use an architect, or at the very least a building surveyor. The Heath Robinson (cartoonist famous for his complex inventions that achieved absurdly simple results) kitchen-diner is not a good look if you want to add value to your home.

Planning it out

If you asked 100 people what they'd most like to have in their kitchen-diner, they'd say an island unit. So says Susan Last, a designer for up-market German supplier Bulthaup. 'An island unit works on several levels,' she explains. 'It looks good and is great for storage – very helpful if you have lost hanging space by knocking through walls to create your kitchen-diner. Plus, if you allow at least one metre between an island unit and the next cabinet, it becomes both an ergonomic workstation and a natural congregation point.'

And congregation is the raison d'être of the kitchen-diner, says Poggenpohl's Robert Laurie. 'In many ways the kitchen-diner is an antidote to our increasingly solitary lives spent in front of the computer at work and, in the case of kids, surfing the net in the bedroom,' he says. 'There is an ever-growing desire to spend more time as a family and a well-designed kitchen-diner, one that encourages socialising, can satisfy that desire.' Having an internet point, plasma screen and other non-kitchen activities in the room can all help conviviality, but one of the most off-putting things about hanging out in the cookhouse is, surely, the lingering smell of last night's bouillabaisse. No longer an issue, says Laurie, who says that steam ovens and proper ducting – together with the most expensive fan you can afford – have made boiled cabbage-air kitchen history.

Decor

As well as encouraging sociability, an island unit can also act as a subtle divider between a kitchen-diner's cooking and living room areas. A bookcase can perform a similar partition trick, as can sliding doors modelled on a shoji, the traditional Japanese room divider made of wood and rice paper. But don't fret about visual and acoustic separation. What you are really trying to create in a kitchen-diner is a seamless whole, and the most effective way of achieving that is with materials.

To this end, Last says, there has been a move away from clinical, hard-edged looking kitchens, and towards softer, more textured woods that look good in a living room as well as next to the dishwasher. And it also explains why, last year, up-market Italian manufacturer Miele started designing dining furniture. Matt lacquer in muted, earthy colours work best, says Last, with bright colours best kept for accessories such as trophy pans, coffee grinders and salad bowls, and the odd stool. In fact, funky seating in an unusual material or hue is a good way of creating a break-out zone in your kitchen- diner. 'Even if it's a very small space, people make a beeline for it,' she notes.

Meanwhile, if you love metal consider dark aluminium which, under the right lighting, has a soft finish. Indeed, according to architect James Gorst, judicious zonal lighting is the key to a successful kitchen-diner. 'Soft perimeter lighting, partition lighting over the island unit – it's all vital,' he claims. 'You don't want to be overwhelmed by the chip pan when you're eating. The aim is to have a less functional approach to food.'

It might be, but don't prioritise form over function. Yes, you want a friendly space, but you also need a room that can withstand the slings and arrows of family life. To achieve this, Evans recommends that your dining table is made from the same material as your worktop, and that seating is covered in robust materials like leather or microfibre cloth.

And if your kitchen-diner is at the back of your house, go for furniture designed for inside and outside use. Italian kitchen manufacturer Dada's glass and
aluminium table Jei is colour-fast and can withstand temperatures of -15 ̊C. Come winter, it can get pretty chilly in the hills of Tuscany, as well as the north downs of Kent.