This three-storey, detached Victorian villa had remained largely unchanged since it was built, except for one significant detail: this once luxurious family home had been subdivided into four cramped and poorly laid out flats.

For years, our clients had dreamed of reuniting the flats to bring back the building’s grandeur, and had been slowly acquiring the flats as they went on sale. When they reached out to us, they had finally purchased the final of the four flats – putting their dream home within reach.

But in all those years of waiting, our clients’ ambition grew and grew. Instead of just restoring the house to its former glory, they decided to go far beyond by adding various extensions and an enormous basement that would house a wine cellar, gym, sauna and the longest indoor private pool in London.

Planning departments are hesitant to permit the conversion of multiple dwellings into one due to the loss of housing. However, in this case, Merton Borough Council approved of our proposals as the building was originally a single dwelling, and our high quality design would provide undeniable improvements to the liveability and longevity of the home.

Designing and building the extensions were well within our usual repertoire. Matching materials and aesthetics ensured that all additions blended seamlessly with the original fabric, and by filling in gaps in the structure, each extension was a sensible addition that looked as if it had always been there.

Far less simple was the basement. Because the basement would house a pool, it would need the equivalent depth of a double-level basement. An excavation of this scale could only be achieved by piling, a painstaking process of drilling into the ground and filling the holes with concrete to form a wall of pillars to support the structure – difficult at the best of times, even more-so in a tightly packed residential setting.

It took three diggers (and the required cranes to lift them back out) plus some of the largest steel beams we’ve ever used to complete the basement, not to mention the logistics of ventilating, draining and waterproofing a 20 metre long underground swimming pool in a 25 metre excavation.

But when you’re in this unique space, it’s clearly worth the effort and the many tons of soil. Looking out over the pool through a glass wall is a fully equipped gym, and on the opposite end a full width roof light allows the sun to pour through a vast glass floor in the garden, making the basement feel surprisingly bright and airy despite its depth.

Most of the basement is hidden beneath the garden, but a small section extends beneath the existing house, providing a route down from above and quick access to a well stocked, climate controlled wine cellar.

Above ground, the plan to unify the home was a tremendous success. The Victorian villa was reborn with contemporary sensibilities, with most of the ground floor opened up into a grand, open plan kitchen, dining, sitting and entertainment space, made even more spacious by the bay-windowed extension.

On the first floor, the master bedroom gained a luxurious master en-suite in addition to its existing full-size dressing room thanks to an extension, while reconfiguration and extension of the roof made space for extensive storage and yet another bedroom and dedicated guest space, bringing the total number of bedrooms to five – all with their own en-suite.

From the outside, you would never know anything had changed. But inside, our clients and their children enjoy a life of absolute luxury, with everything they could ever need and more all under one roof – and beneath the garden, too.