THE Prince of Wales is building a new home fit not so much for a king as a daughter-in-law. He has put in plans to build a country house that some say would be a suitable “starter palace” for a young couple looking to settle down.

The six-bedroom classical mansion is to be built in an isolated valley on the Welsh border, less than an hour’s drive from Highgrove House, his home in Gloucestershire.

Although the palace maintains the property will be rented out commercially, the plans will fuel speculation that Prince William is being “kitted out” with his first country home in preparation for marriage.

Rumours that William will soon propose to Kate Middleton, his girlfriend of three years, were fuelled by her appearance at his passing out parade at Sandhurst on Friday. She and her parents were accompanied by Jamie Lowther-Pinkerton, the private secretary to Princes William and Harry.

According to lip-reading experts, Middleton, 24, said of her boyfriend: “I love the uniform. It’s so sexy.”

The event marked the first time Middleton has been Prince William’s guest at a high-profile event attended by the Queen.

Robert Lacey, the royal historian, said: “All the signs are that the senior members of the royal family not only approve of the girl but approve of the way the relationship is developing — rather cautiously.”

During William’s leave from Sandhurst the couple have spent much of their time at Middleton’s flat, owned by her parents in a block in Chelsea. They became flatmates while students at St Andrews University in 2002.

Ever since the Duchy of Cornwall acquired Harewood Park, a 900-acre estate in Herefordshire, six years ago observers have believed it will become the country seat of Prince William when he eventually marries.

In 2004 the duchy submitted plans for a 14,855 sq ft mansion on the banks of the River Wye, which it said was destined for the letting market. New plans have now been submitted by Craig Hamilton, Prince Charles’s architect, for a “starter home”, better suited for a young prince anxious to avoid ostentation. It will be a two-storey, six-bedroom home of 8,500 sq ft.

The house is designed to be as eco-friendly as possible. Solar panels are planned for the roof, natural sheep’s wool will insulate the loft, and recycled bricks will be used for the walls. “Salvaged” Welsh slate will be used for the roof.

Central to the new plans for Harewood Park, lodged with Herefordshire council, are a series of large arches that dominate the front and back of the house and frame the windows. In another nod to antiquity, friezes of winged lions supporting a lyre will decorate the outside walls.

Inside, the house will boast a double-height central hall dominated by eight Ionic columns. The four-bay garage to be built under a terrace is inspired by a 4th century BC Greek monument on the southern slopes of the Acropolis.

There will be an orangery and a chapel in the grounds is to be converted into an artist’s studio.

Should William and his future wife move in, he will be required to obey a strict regime of recycling and energy conservation. A “sustainability report” accompanying the plans states that energy can be saved if “showers are taken instead of baths” and “low flow showers and spray taps are fitted so that less hot water is required”.

“Energy efficient light fittings” and recycling bins are included in the scheme, while trees are to be planted to compensate for pollution created by builders’ lorries driving to and from the site.

Stone for the building will come from a quarry on the estate within a mile of the house.

A spokesman for Prince Charles said the previous set of plans for a larger house at Harewood had been shelved because the projected rental income did not justify the financial outlay. “The property has always been one the duchy wanted to let. Scaling it down makes it easier to let,” he said.

Earlier this month Middleton made her first public appearance at Sandringham — on previous visits she has taken care to avoid being photographed — and she has reportedly been invited to Christmas lunch there, an unusual honour for a royal girlfriend.

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