It is not easy to get anything new built round here. Everything has that "Royal" tag, is watched over by the gilded statue of Albert, Prince Consort to Victoria, in his immaculately restored memorial. Consequently everything comes with a heritage warning label. Look at the flak Sir Nicholas Grimshaw has run into designing an extension for the Royal College of Art on the far side of the Royal Albert Hall. Look at the furore over Kathryn Gustafson's excellent Princess Diana Memorial Fountain (not a fountain, but a watery landscape), nearly complete nearby in the ineffably royal Kensington Gardens. And the RGS is very prominent, right on the corner of Kensington Gore and Exhibition Road.

But the RGS has done something very intuitive on its plot. It has worked with an unexpected and previously largely untried architect from a younger generation, Craig Downie. They commissioned him purely on his response to the building and its context. And he has not let them down. This is only Downie's second significant public building project (the first was a masterly little woodland pavilion for Sculpture at Goodwood down in Sussex). He has been living the RGS job for some years, doing things such as completely re-ordering its big public lecture theatre. Now he has also added new buildings. They are an object lesson in modern contextualism.

Here was the problem. The only place the RGS could expand to build a new public gallery and reference library was out the back. But that was the garden. And nobody wanted to lose the garden. Nor did it make much sense to have the hoi polloi tramping through the corridors of the old building to get to the new one, especially as that would mean making an offputting loop round the corner from Exhibition Road anyway. So what Craig Downie did was make a new entrance at the top of Exhibition Road, set his exhibition pavilion right there, and then tuck everything else under a terrace up against the old building, sinking it into the ground to make it part of the garden. This all cost £7.5 million ($13.8m), greatly helped by the Heritage Lottery Fund, and it works. QED. As with most good buildings, it all looks remarkably simple. It was not, at all: this was a complicated bit of three-dimensional cutting and pasting.