California dreaming: Why an entrepreneur who wanted to build an LA-style Modernist pad took a leaner, greener route

Nigel Mitchell had a very particular kind of dream-home in his mind. For more than a decade, this Bath entrepreneur plotted to demolish his existing house on a site overlooking Bath, and build an ultra-Modernist home on the same site. But delays, legal disputes, and surging steel prices pushed the projected construction cost to more than £1m, and he was forced to think again.

He's in a very different kind of dreamland today. The wrecking ball wasn't needed, after all. Instead, his home has been utterly transformed by Dow Jones Architects, one of Britain's most gifted younger practices – and at about half the cost of the original project. So-called "retrofit" design can rarely have produced such a strikingly original architectural outcome.

The energy-saving wall and roof U-values are now more than three times more effective than before. "This house is really exposed to south-westerly winds, but now very little heating is needed," says Mr Mitchell. But the story of the design is not about architecture as if it were an eco-warrior's surgical strike. It's about the way bold ideas are shared; and it recalls a comment made to me by the legendary architect Frank Gehry. What, I asked him, was the key factor in design development. "The client," he replied, instantly. "Great architecture can't happen without a great client."

Nigel Mitchell was just such a client. "I have a real enthusiasm for good design in architecture and products," he told me. "It's a passion, and building my own house has always been one of my passions. I knew that I had to find an architect whose work I loved. I don't see any point in finding an architect and getting them to do only what I want. You have to trust their ideas."

He set up a design competition between three notable architects, but it wasn't even close. The scheme presented by Alun Jones and Biba Dow of Dow Jones Architects was "head and shoulders above the other proposals in understanding the brief, and they absolutely got what I needed".

There is a second California connection. When Jones and Dow, who both studied architecture at Cambridge, presented their scheme they showed examples of the iconic Californian Case Study Houses of the 1950s and 1960s, designed by Modernist masters such as Richard Neutra, Pierre Koenig and Craig Ellwood. Though fiercely pared down in its use of glass and steel, this architecture maximised the relationship between houses, topography and views. Their interiors, coupled with the positioning of the buildings, set up superb outlooks and created open, light-filled internal spaces.

It was this ideal, coupled with low-energy performance, that drove the redesign of the Bath house. Dow Jones's grasp of environmental performance was informed by their design studies for waste-burning energy plants for the Greater London Assembly and Design for London; and by their collaboration, with other designers, on the London Development Agency's Green Enterprise District. The practice's housing projects – ranging from a new house in Walberswick, Suffolk to a two-level apartment for a film director in an ex-Victorian factory in London – also show an ability to celebrate materiality.

In the Bath project, they worked with Momentum Engineering to cover one kind of material – the building's original faux Bath stone blockwork and roof – with something quite unexpected: a heavily insulated envelope of crisply seamed zinc, pre-patinated to a very dark bitter-chocolate brown. Voila! Large but rather uninteresting executive home becomes the most sharply tailored architectural Mod in Bath.

How was it done? The house was originally organised around an upper entrance level, with a living room, dining room, kitchen and garage; four bedrooms and a bathroom were on the lower ground level. Yet only the bedrooms opened directly on to the garden, and the route from living room to garden was through the kitchen and utility room.

"There were organisational and functional problems with the house," says Jones. "But the major architectural issue was the way the building related to the topography. It was obvious that the house was in absolutely the wrong place – in the middle of its site. So there was no effective relation between the internal spaces, and the garden and views. There was no sense of threshold."

Dow Jones created a new studio and garage block next to the main house, with the ground floor studio opening directly on to a courtyard garden – and the view between the two buildings frames the vista across the valley towards Englishcombe. The polished concrete of the studio floor continues seamlessly outward to form the surface of the courtyard, which is linked to the house via a sculptural concrete stair which leads up to a new balcony that runs across the south elevation of the main house.

"We felt the garden should be an immediate presence in the main house," said Jones. The entrance level was reorganised so the living room faced south. One original internal wall was removed, and the dining room repositioned on the western side of the house so dinner could be eaten with the setting sun projecting the shadows of the trees on to the dining room wall through a new full-height window. The kitchen is set on a polished concrete slab that extends out of the house and runs into the garden; and a projecting trapezoidal rooflight in the kitchen ceiling casts the morning sun on the shining surface of the floor.

"The house is now connected to the site, physically and in terms of how the passage of the sun around the house is structured as an internal experience," explains Jones. "The instant you walk in, you see straight through the house and see the garden. If you turn 90 degrees to the left, there are now clear and expansive views of Bath and the landscape. The upper level of the house feels canopy-like, and this emphasises the reconnection of the topography with the building."

The execution of the design, says Mitchell, has been "exceptional". This is not surprising: Dow Jones is well known for its almost obsessive approach to design details, carried through in this case by the project architect James Grayley. For example, great care was taken to ensure the zinc skin formed a crisp vertical upstand on the gable ends; most architects would have let the builders bodge the zinc downward over the edge of the gable-ends – easy and effective, but aesthetically crude. Another small but masterful detail is a "secret" guttering system that helps to ventilate the interiors.

And what might the ghosts of Neutra, Koenig and Ellwood make of Prospect House? They'd want much more glass, of course, a flat roof, and trays of martinis. But they might also concede that Dow Jones's transformational project is a new kind of Case Study House for the 21st century – one that proves that environmental design can be as sharp as a tack, rather than the architectural equivalent of a worthy hair shirt.