It was an inauspicious start. I arrived at a sea of car parking, at what I thought was a front door to the large and sprawling web of machine-like mechanisms – wrapped, almost apologetically, in dreary skins – that pertain to be architecture. My taxi driver asked if we were near an entrance. The member of staff asked if I was well enough to walk. Having explained I wasn’t a patient, I was directed to a well-concealed door that led to the children’s cancer ward. And so I found my way to the architects and client who were waiting to receive me in the light-filled and surprisingly buzzing lobby of the Oak Cancer Centre.

My first request was that we exit the building, so I could discover the way their new building should have been approached. As we walked away, we met more architects – from another practice – who were there to be interviewed for a new masterplan. They assumed that myself and Benedict Zucchi and Dominic Hook from BDP were competing for the same. We were not. I was the interloper, there to review the building, whilst Benedict and Dominic are the architects for both the Oak Cancer Centre and the new hospital, just starting on site, that will help define the western gateway from the suburban streets. BDP have already produced a masterplan inspired by the potential to expand the Oak Centre, to engage their first building with their second, and to engage both with the other recent buildings that, until now, have been slotted like new teeth into a chaotic mouth free of orthodontic design.

I suspect I’m not the only visitor to turn up at the wrong place. It’s not the fault of the architects or their clients or their fine building. But it is a sad tale of opportunities missed – in the past, now and, despite the rumours of an impending masterplan, I fear into the future. Why? Because healthcare design is mired in the complex and absurd. Everything – buildings, operations, services – is endlessly undergoing restructurings and reorganisations defined by rules unique to the system. Most buildings within estates are built for a specific purpose, become redundant, and are left to decline before being removed. And the cycle starts again.

In healthcare, the call is always for the new, which is understandable when you see the state of some old and not-so-old buildings – most definitely buildings not architecture – in which staff work and patients wait. But this mentality is out of synch with a world where re-use and reinvention have moved from buzzwords to become a pressing and exciting reality. Where buildings are conceived as generous containers for the dynamic theatre of everyday life: legible, adaptable, usable, delightful, and even Architectural. Happily, the Oak Cancer Centre is of this world. A building of scale mined out of the complex and confusing little city that has grown up around The Royal Marsden Hospital and The Institute of Cancer Research.

The building sits well in its carefully landscaped entrance – which I hope is a sign of the site’s future. We are greening our cities and the need to green our hospitals seems so obvious that it feels absurd it should need to be stated, especially when the building sits in leafy suburban Sutton. This important new landscape will fast mature and help, not only those who use the two entrances at lower and upper ground, but all those walking through and past the site. The lower entrance is to diagnostics, which the architects have cleverly designed as a separate house-within-the- house, on the basis that, once tested, many of those visiting may turn out to be free from cancer. This separation from those who may be undergoing treatment is pragmatic in terms of managing both infection and trauma.

The upper-level pedestrian entrance – the place I left from, and to which I would have been happy to arrive – has a taxi drop-off which is vital for outpatients receiving regular chemotherapy treatment. The gentle curve of the façade welcomes all comers, and is free of the essential but industrial service vehicles that are allowed to dominate too many hospital entrances. There were no smokers either. Nor any need for yellow lines to guide us. Instead, the architecture of shadow, canopy and recess was doing its job of directing us to where we wanted to be.

Once inside, there is a very public and welcoming linear anteroom with café and receptions that links to the other buildings beyond. Through this anteroom is the business end of the hospital, another unusually generous urban space. A square or grand room – similar in principle, if not detail, to the space Charles Barry created at The Reform – it suggests the idea of a club. Though The Oak Cancer Centre is a club whose membership is involuntary it still has that comfortable feel. The atmosphere is set by the volunteers – badged as such – who give their time to reassure, welcome and guide outpatients. There is a buzz to the space because it is a single, memorable, and understandable place for many different people to do many different things: to receive treatment, to give treatment and to research the treatments that seek to turn cancer from a life-threatening condition into a chronic illness.