The most ambitious series yet? Grand Designs returns with a toxin-free 'health house' and a family home built entirely from concrete

A cutting-edge 'health house', a home built entirely from concrete and identical stilt houses for identical twins are among the extraordinary self-build projects to look forward to in the 18th (yes, really!) series of Grand Designs.

Presenter Kevin McCloud will be back next Wednesday, September 19 to guide us through seven new episodes, following passionate people determined to make their dream homes a reality.

Inevitably, nerves and patience will be tested as each family battles with spiralling costs, contractor problems and the elements refusing to play ball. In Kevin's words, there's more of the "ambitious, crazy, wholesome fare that makes Grand Designs what it is".

"The majority of people we film have a degree of self-belief, that’s what's got them there in the first place," he says. "That brings with it a belief that I hear again and again, which is, 'This project is going to come in on time and on budget.' My answer is, 'I've never really seen a project do that.'"

So what can we expect from each episode?

Burnt-out folly

The first episode takes us to Aylesbury Vale, an agricultural area in the northern half of Buckinghamshire.

Here, Spanish architect Jimmy and his wife Mimi are embarking on an epic mission to convert a Grade II-listed folly into a family home.

Gutted by fire and with crumbling stonework, it throws up a wealth of issues – and that’s before they realise it's built on an Anglo-Saxon burial ground. Oh and just to ramp the stress up further, they've got a baby on the way.

Surf's up

In the second episode we meet Harry and Briony, who have left London behind for the rugged coastline of north Cornwall.

Not content with a big move, this adventurous couple are dead set on building a radical steel and glass surf house inspired by James Speyer's Ben Rose House.

The Illinois modernist classic famously featured in cult Eighties film, Ferris Bueller's Day Off. Needless to say, they won't be having any days off themselves...

"So many people know that movie," says Kevin. "Our couple had never gone to Illinois to visit [the house], so this programme ends up as an interesting lesson on what happens when you pursue an idea on a whim."

The first Grand Designs 'health house'

Born and Elinor's plans have a serious motive behind them — both their sons suffer from life-threatening allergies and are often in and out of hospital.

As such, they must navigate a niche set of challenges to ensure their new Richmond home is as safe as possible.

In a show first, their 'health house' will be built with low toxin materials and feature a mechanical ventilation system that filters the air. The couple have Kevin firmly on side – he was diagnosed with asthma aged 16.

"I'm very careful about the air quality where I live and anywhere where I like to stay, but for these guys, it's a totally different ball game," he says.

"They've used solvent-free, low-VOC paint, they've used a minimum number of glues and the MDF in the building only contains trace elements of formaldehyde and no varnishes.

"They've produced a house that's beautiful – glorious – it just also happens to have been built with health in mind."

Concrete crazy

Over in Lewes for episode four, Adrian and Megan are busy realising their dream for a family dwelling built entirely from concrete.

Adrian has been obsessed with the hard grey stuff since his time as a semi-professional skateboarder in Aberdeen, but it remains unknown whether such a cold, minimalist material will offer the right environment for bringing up their three young children.

Identical twins Nik and Jon live identical lives in many ways. They work together, drive identical cars, have matching dogs and now, they’ve decided to build two identical modern-industrial homes next door to each other.

Their twin houses are perched on stilts with giant floor-to-ceiling windows, maximising the magnificent views across a picturesque mill pond. Why change the habit of a lifetime?

"This is an exercise in getting stuff right, having two tries instead of one," says Kevin. "Two minds are better than one, a joint creative strategy, and trying to get the best out of a pretty low budget that they both have. I like that very much."

Return to Leominster

Steph and Alex's modular-style black barn-house is set in an old gravel pit on the site of her grandparents' farm.

Steph had been longing to return to Leominster in Herefordshire for 20 years before finally moving back.

Her architect is her oldest friend – they played here as children – and together they have built an agricultural-inspired house with a corrugated steel roof and aluminium cladding. From the road, it looks just like a traditional barn…

Catching-up with the King of Cob

This series' final 'revisit' takes us back to Kevin 'King of Cob' McCabe's, 'cob castle'.

A master in the ancient art of cob building – wrestling houses out of mud and straw with his bare hands – Kevin didn't want to build any old cob house. He set himself the challenge of meeting the highest environmental performance targets ever set.

In 2013 when his original episode was broadcast, Kevin's house didn't even have windows. Five years later, he has finally finished his soil and straw masterpiece.

Grand Designs returns at 9pm on Wednesday September 19 on Channel 4