They already ran a business together, dressed alike, drove the same cars and even owned matching dogs.

Then inseparable identical twins Nik and Jon Daughtry took their extraordinary bond one step further - by building two striking Grand Designs homes next to one another.

When their ambitious project appeared on the hit Channel 4 show in 2018, presenter Kevin McCloud introduced the Sheffield siblings as 'identical twins with near-identical tastes'.

But, eight years on, something is finally set to separate Britain's most intertwined brothers: a For Sale sign.

Despite once declaring that he had 'no desire' to ever leave their remarkable twin development, Jon has placed his spectacular five-bedroom home on the market - leaving Nik in the near-matching property next door.

The brothers, now 56, spent nearly three years battling delays and a spiralling £610,000 bill to create matching black-clad industrial-style homes on the site of a derelict 200-year-old corn mill in Oughtibridge, South Yorkshire.

One property became numbered 14A and the other 14B. Even the adjoining mill pond was divided precisely between them, with each brother owning one half.

'We are just so close, so alike. He's my best friend,' flat-cap-wearing Jon told viewers. 'You can't argue with the mirror,' Nik, of equal sartorial style, joked of their relationship.

It is 14A which is now up for grabs, with an estate agent inviting offers 'in excess' of £1million for the steel-framed home, which stands on stilts above the remains of the original mill machinery.

The listing immediately sparked concern among Grand Designs fans, who wondered whether the sale signalled trouble.

One wrote online: 'This was the two brothers who built next to each other. I wonder what's happened here. Hope they're both OK.'

Another added: 'I bloody loved this particular Grand Design. It's sad it's not one of the twins' forever home.'

The siblings, who run a design agency and a restaurant business together - and have both married women who appear strikingly similar - batted away suggestions of a brotherly bust-up.

Jon told the Daily Mail: 'It's still very much happy families - absolutely.

'My brother's fine. He's releasing me - he's letting me go.

'We still work extensively together, so it's not as though we're disappearing from each other's lives.'

Instead, Jon said he and his wife Alison were searching for another Grand Designs-style challenge, ideally involving a smaller rural building in need of renovation.

He said: 'I think I've got another build in me somewhere.

'My wife's from farming stock, and we'd like to get a little bit back towards that.

'Hopefully it'll be a bit of a ruin that needs renovation, and that will give me an opportunity to create something a little special on the inside.

'But it won't necessarily be a building similar to the one that I'm already in.'

The brothers were already accustomed to living at extremely close quarters before their Grand Designs fame.

The pair occupied homes only a few doors apart, drove the same Mini Clubman car, had matching Golden Retrievers and were said to be 'constantly moving' between one another's houses.

So when they discovered a derelict former corn mill with an overgrown pond, they decided to pool their resources and build two new family homes on the site.

Grand Designs viewers watched on as the ten-month project inevitably ran into trouble.

The construction schedule stretched towards three years and the final bill climbed from a projected £345,000 to £610,000. The brothers both had to ask for another £70,000 each from the bank in order to finish what they had started.

When McCloud returned to inspect the completed homes in 2018, he was taken aback by the transformation.

Inside, exposed concrete floors, steel staircases, bare block walls and visible copper pipes created an uncompromising industrial appearance.

McCloud compared one of the homes to 'a very luxurious power station' and described the overall development as 'absolutely epic'.

Jon lives in his home with his wife, Alison Jarjour, and children from a previous relationship. Nik occupies the neighbouring property with his wife Emma and their children.

Jon previously told Grand Designs Magazine: 'Ali and Em shared in the dream, participated in the nightmare and ultimately got us through.

'While much of the story has been about the relationship Nik and I share, this journey wouldn't have been possible without them. Behind every great twin there is an even greater partner.'

At the time of the programme, Jon appeared certain that the brothers had created their permanent home.

'We've no desire to leave this special place,' he said.

'The journey was great, the destination fantastic. Now we plan to enjoy it.'

But the property is now being marketed as a rare opportunity to own one of Grand Designs' most memorable houses.

Estate agents describe 14A Church Street as 'a true triumph of industrial design'.

The listing says the five-bedroom detached home was created with 'a sympathetic nod' to the history of the site with a layout 'designed for modern family life'.

Estate agents Blenheim state: '14A Church Street presents a rare opportunity to acquire a spectacular five bedroomed detached residence on the historical site of Oughtibridge's former corn mill.

'Thoughtful layout planning and selective material choices have enabled effortless living throughout the property, with a focus on accommodating modern family lifestyles.'

The house includes a vast split-level kitchen, floor-to-ceiling glazing, a mezzanine lounge and a principal bedroom with a freestanding bath overlooking the trees.

Outside, stone steps lead down towards the shared mill pond and a timber jetty.

Jon has previously hinted that life at the secluded property is not always entirely peaceful.

Last October, he launched a furious Facebook tirade at 'selfish f***ers' who set off fireworks weeks before Bonfire Night.

'You're two and half f***ing weeks too early,' he wrote. 

'If you had nervous dogs like I do, you'd understand how throughly f***ing troubling this is.

'I can't medicate my dogs for two-and-a-half weeks before and two-and-half weeks after the 5th. You f***ing morons!!'

Both houses have also been used as backdrops for television productions and commercial shoots.