A castle is for sale. Guess how much? It’s in a picturesque Bucks village in the foothills of the Chiltern Hills. Guide price: £850,000.

Dinton Castle is Grade II* listed. It was featured on the first programme of the current series of Kevin McCloud’s Grand Designs.

In historical and architectural importance, it’s only one notch below Grade I.

Roughly speaking there are 500,000 listed properties across the country. Ninety two per cent are Grade II. Grade II* listing accounts for 5.5 per cent. Only 2.5 per cent are Grade I.

The folly in Dinton is one of 357 Grade II* listed buildings in Bucks. It has been there for 250 years. It was built in 1769 by Sir John Van Hatten as an ‘Eye Catcher’ to house his collection of fossils and ammonites.

What has become a local landmark was a wreck when it was bought by a Spanish architect and his English wife.

Jaime and Mimi Fernandez have fully renovated and meticulously restored the folly to create a remarkable home.

The rooms are spread out over three octagonal floors with an abundance of charm and character.

As the agents at the Great Missenden office of Hamptons International point out the exposed limestone walls studded with hundreds of ammonites give the interior a unique atmosphere.

The architect has capitalised on the nautical origins of the structure for their choice of decor that runs through the building.

“The ammonites were sea animals. They were around in the Jurassic period, the era of the dinosaurs.”

The blue of the Moroccan floor tiles in the kitchen gives the impression of a boat on the water, the designer suggests.

The circular room has a high ceiling, the gas range cooker is set into the original fireplace, the worktops are sold wood. A passage leads to the utility room.

Nautical rope for a handrail on the stairs continues the marine theme.

On the first floor is the sitting room. There’s another fireplace here, more oak beams, the ceilings are high throughout the folly, the floor is oak and the views over the surrounding countryside in every direction are wonderful.

A glass door at this level opens onto a terrace with steps down to the garden.

On the second floor are two bedrooms each with en suite bathroom – one has a shower cubicle, the other has a bath.

Underfloor heating from top to bottom of the building keeps you cosy in winter.

On the top floor is the roof terrace enclosed in the original parapet. Imagine the 360-degree views from here.

Jaime says his wife first noticed the folly. She spotted it in its dilapidated state on one of her trips back to England to visit her parents who live nearby from the home she shared with her husband and their baby son in Spain.

“As soon as I saw it, I could immediately visualise what a beautiful house it would be,” Jaime says.

“It took a year for us to get planning permission and another year and two months to build. In the middle of it, my wife gave birth to our second baby.

“It was a very exciting time for us.

He added: “English Heritage were immensely helpful.

“We got on very well with the heritage officers. For me as an architect it was an Anglo-Saxon project.

“I’d never been involved with anything like it before, not in Spain nor here.”

Their two small sons have learnt to manage the four flights of stairs without a problem.

George now aged three and a half and Lucas, born a year and four months ago have the makings of admirable crew members for a tall ship.

How did they get on with Kevin McCloud, the presenter of the chart-topping Channel 4 series on home design?

“Very well, he’s a very nice gentleman.”

What next for team Fernandez? “We have no clue.”

The young family are the same as most others. They need to be free to sail where the work is.

For more details about Dinton Castle, call the agents on 01494 863134.