The most important thing I know about architecture, I didn't learn at university.

I learnt it in my first ever architecture job, when I was by far the most junior person in the office. And it came from Professor Sir Colin St John Wilson, or Sandy, as he was generally known.

He was a proper architect of the old kind: a big drawing board, everything drawn by hand, and probably never having sent an email in his life. And really, this shouldn't have been a surprise. He had run architecture at Cambridge for decades. He had a way of imparting knowledge that very few others have.

At the time, he was working on a design for the Royal Academy Schools, and I was helping with the masterplan for the whole of Burlington House, colouring up drawings for reports showing what function went where. The sort of work done by Part 1s.

He could have just handed the sketches over to me, most people would have. He didn't have to explain any of it.

Instead, he called me downstairs and I stood beside his drawing board as he began, to my surprise, to explain the scheme.

And he didn't start with a room or a function.

He started with a person.

He told me about the person who organised the Royal Academy's Summer Exhibition each year. Thousands of works arriving, to be recorded, cared for, stored, shown to the panel, sorted, and then either hung or sent back.

An enormous undertaking, largely understood through the work of one person.

That person needed somewhere for things to arrive. Somewhere to store them. Somewhere to move them. Somewhere to show them. Somewhere to work out what went where.

The Royal Academy has grand courtyards, beautiful entrance spaces, exhibition galleries, cafés and undercrofts. But the story Sandy told me about the masterplan wasn't really about any of those things.

It was about one person trying to make the Summer Exhibition happen.

And something clicked for me that day.

I realised that the best way to understand a building is often to understand the people who make it work.

And, perhaps more importantly, I realised that what a building was already doing mattered more than what I thought it ought to be doing.

That sounds obvious now, but it was quite a significant shift in how I thought about architecture.

There is something very seductive about the idea of the architect as the person with the big idea: the gesture on the back of a napkin, the beautiful concept, the clever plan.

But Sandy was showing me something different.

The architecture was in the detail of how the building needed to work. In the thousands of small requirements that came from the people using it, and in the relationships between them.

The job wasn't necessarily to impose an idea on the building. It was to understand what was already there, and then work out what the building needed to become.

I went travelling soon afterwards, and Sandy very sadly died. The project was eventually handed over to another firm, so I don't know what, if anything, happened to his masterplan designs.

But I've thought about that conversation so many times over the years since.

A building isn't just its fabric, or its plan, or its history.

It is a place where people do particular things.

And those things matter.

The granular detail of how a building is actually used — what people need from it, how they move through it, what has to happen where, and how all of that changes over the course of a day, a week or a year — can seem like a small thing.

But it gives you a much better basis for deciding what a building actually needs.

Twenty years on, that's still how I approach buildings.

Before I start asking what we can change, I want to understand what the building is already doing.

That's why I set up the consultancy.

Understanding how a building actually lives, and who makes it work, gives you everything that follows.