When Rosemary Harris and her husband Raymond bought Somerset Lodge back in 1987, its derelict, steeply sloping, half-acre garden did not look promising. There was little in it, apart from a few miserable- looking orchard trees, and the walls within were collapsing into the slope.

Undeterred, the Harrises, both architects before they retired, went ahead with the purchase of the Grade II*-listed, 17th-century house in Petworth, West Sussex. For two years, as they worked to restore the property, Rosemary came down from London every weekend to dig out stones from the garden.

“Being at the drawing board in London and coming down here to flex my muscles,” she says, “I couldn’t wait.

“I love moving earth,” says Rosemary. “I’ve got about 20 ways of moving it.”

The Harrises rebuilt the wall and created a large lawn, the only non-sloping part of the garden. Underneath the soil in the orchard they discovered the rubble of former houses, and this they recycled to create retaining walls, thereby staggering the slope into more manageable areas.

Garden enthusiasts can see the results this Saturday, when the Harrises’ garden opens to the public as part of the National Gardens Scheme’s Chelsea Week Tours.

It deserves a visit. Today, there is a box parterre for fruit and vegetables, a large border filled with wild flowers, areas for roses, two ponds, a shrubbery and four lawns. Despite being in the centre of Petworth, the open aspect at the rear of Somerset Lodge leads you almost to believe that you are in the country. Beyond the garden, with the advantage of being slightly higher than the parkland it abuts, thanks to the buttressed outer wall, there are long views of “borrowed landscape”, stretching, on a good day, to the North Downs 14 miles away. “Every step of space has a view,” says Raymond.

It was Rosemary who designed the garden, but, in its structures and furniture, Raymond is very much in evidence. His handicraft, including the garden furniture, is made up in his workshop — two 16th-century former cottages in the garden. At the far end of the garden, for instance, is a gazebo, which he modelled on the stone fountain in the great court of Trinity College, Cambridge. With its lead roof topped with a gold-leafed ball, it provides both a focal point for the garden and a picturesque place in which to sit and admire the landscape beyond.

This is a garden in which everything is thought through to the smallest detail. The balustrades around the main lawn, for instance, were made by Raymond and are based on the magnificent four-floor wooden staircase inside the house. When the couple decided to add extra beds in the parterre, they created them as mirror images of the beds that radiated from the conservatory.

When designing the garden, Rosemary drew up a plan. “As I did the work, the plan changed and it became more structured,” she says. Even the wildflower area has been given formal perimeters, in the form of a rugosa-rose hedge on three sides and a box hedge by the upper path. On the other side of the path is a copy of a Roman statue of Mercury — the original was found during excavations at Moorgate, London, for a new building, designed by Raymond, and is now in the Museum of London. As it is in a shady area, it is surrounded by ferns and fatsias — “they provide the fig leaves,” says Raymond.

Rosemary’s practice specialised in the restoration of old houses. Yet although the garden is in keeping with the spirit of the house, it is not a slavishly “period” garden. One touch, though, you suspect the 17th-century owners would have recognised and approved of: in the front garden are two box parterres spelling out R, the initials of today’s inhabitants.

Tickets for the Chelsea Week tour to Somerset Lodge and three other Sussex gardens are £68 and include travel from Surbiton station and a picnic lunch. Call 020 8339 0931

Explore Property

• Buying & Selling

• New Homes

• Overseas

• Interiors

• Investment

• Gardens

• Cool In Your Code

• Specials

Times recommends

• Analysis: frustration for everyone - except investors
• Spanish property: why it's crunch time on the Costa del Sol
• Buy-to-let: professional investors cash in on the credit crunch