When Luke and Charlotte Tozer learned they were expecting their first child, they knew it was time for a bigger house. Luke, an architect, was not averse to a challenge, so they went looking for a building that might need a renovation. What they found might have made other potential buyers flee—–a constricted site in London’s Notting Hill occupied by a derelict 1950s cottage—–but the Tozers used their imaginations to see the potential. “Only an architect would have been crazy enough to buy it,” says Luke, a director at Pitman Tozer Architects.

The front of the cottage was a mere eight feet wide, expanding to the rear where it nestled among the back gardens of neighboring buildings dating from the 1860s. It was immediately clear that obstacles would arise not only in the design process but also in accessibility during construction on this unusually narrow lot. “We had always wanted to build a house for ourselves,” Luke reflects, “but looking at this site, I couldn’t quite work out if it was my dream or a nightmare.”

Though the cottage was in poor shape, having an existing building provided a starting point from which to draw. “With the new design we were able to go up, back, and down,” says Luke. “We dug out the whole back of the site, and we were able to increase the floor area of the new house by about half compared to the original.”

At each stage, the slimmed-down nature of the site required creative thinking to get around access problems, from building a hut for the contractor that could be moved around on wheels to finding a drilling rig narrow enough to reach the backyard to drill the 165-foot-deep boreholes that are key parts of the geothermal system.

The ground-source heat pump, which uses natural subterranean warmth to heat the floors and water, was one of a multitude of measures the Tozers took to make the house as sustainable as it would be beautiful and livable, from overhead to underfoot. On the roof and under the courtyard garden, a rainwater-harvesting system was installed in order to use reclaimed water for the home’s toilets. Materials for the timber-and-steel-frame house were carefully selected from responsible sources, including the wood for the custom staircase, which is sustainably grown larch composite board. Operable skylights in the stairwell and the sitting area allow for natural passive ventilation on hot days, while the orientation of the glass to  the sun maximizes solar heat gain on cold days. Many of the green features in the house are common sense, including high-efficiency glazing and lamb’s-wool insulation.

Ultimately, the Tozers were rewarded for their painstaking process. Having bought the site in 2005, they finally moved in two years later and were just within their $987,000 budget. The finished interior maximizes every square inch of space yet avoids any feeling of claustrophobia. The narrowest, street-facing section of the house is essentially an entrance area on the ground floor, topped by a stack of three bedrooms.

By placing the sleeping quarters in the leanest region of the house, the living zones gain the more expansive back area, which unfolds dramatically into a semi-open-plan kitchen, dining area, and sitting room, enriched by banks of retractable glazing that open out onto a courtyard. The indoor and outdoor floor levels match up for a seamless transition. “We enjoy looking out onto the garden year-round,” says Luke, “but on a nice day, with the gatefold doors open, we have a much greater sense of space. It becomes one big room within this rather Californian indoor-outdoor idea.”

For the Tozers’ two young boys (their second son, Alexander, arrived two years after Mark), the openness of the main floor makes for a great place to play. “It’s very well suited to two small kids who can have the run of the ground floor during the day,” says Charlotte. In order to make it equally suited to adults, built-in storage means toys and clutter can be tucked away after the kids go to sleep.

Their children are one reason why Luke and Charlotte chose to emphasize sustainable design—–both to teach the boys environmental awareness and to keep their carbon footprint small. “With young children we do a lot of flushing,” says Luke. The couple encourages turning off lights and taps but try not to be too overbearing.

One more bonus of the green approach: “It is cheap to run,” Luke says, “but none of that is especially noticeable. It’s about trying to design in a sustainable manner without making a song and dance about it. We did superinsulate the house, and the glazing is very high spec. It’s in excess of what you have to do in the UK, but the benefit is worth it.”

The rainwater-storage system saves on the family’s water use, providing recycled water to flush the four dual-flush toilets. No special allowances were required for the system, which comes under standard UK building regulations. The Tozers considered graywater recycling to reuse bath and shower water, but decided that the simplicity of the rainwater system suited them best. They did, however, install a separate rainwater-collection barrel for watering the courtyard  garden, even though the plants were selected for their low-water needs.

Though the environmental upshots of their home are many, Luke and Charlotte are perhaps most pleased that the unique constraints of the site became its best asset: By having to situate the majority of the living space to the rear, with just a sliver of the facade exposed to the street, the Tozers ended up creating their own private world, turning a dejected lot into a safe haven where their family can grow.

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A Slender Geothermal Cottage in London