Sneak preview: British Museum extension

The last time I took a close look at progress on the British Museum’s new £135m extension, it was 2011, and just a great big L-shaped hole in the ground. And a very deep hole at that: in order to pack a lot of accommodation onto a very tight site next to the Grade 1 listed museum , this is a stealth building – actually a linked group of ‘pavilions’ - that goes a long way into the ground – 18.79m deep.

One of the pavilions, in order to create a breathing space at ground level, is entirely underground. The tallest rise 26m to the cornice level of the Edwardian wing they adjoin. Construction, in the hands of contractors Mace, has been very rapid. Now the centre is structurally complete, though there is a year to go before it opens.

“Thank you for fluorescing,” says BM director Dr. Neil MacGregor as we struggle into the full PPE kit for a tour of the new building – starting in a set of plain Edwardian-era rooms that have not been seen since they were used by the British Library when it occupied the heart of the museum.  These will now become part of the public entry sequence to a big new exhibition gallery at ground level in the new building. You will enter from the north-western corner of the Great Court: new openings have been made in the wall.

This gallery is pretty impressive: 70m long, 16m wide and 6m high, yielding 1,100 square metres of column-free rectangular space, capable of subdivision. But most of the project is about back-of-house stuff.  Where Foster’s Great Court project at the Millennium was all about making public space at the heart of the museum, this part – full title the World Conservation and Exhibitions Centre – is overwhelmingly about the conservation studios and the handling and storage of precious – and sometimes very heavy – objects. Some of these studios, especially the ones up on top beneath glass roofs, promise to be marvellous working spaces. But what gets the museum people more excited is something positively banal: a loading dock.

Bizarre as it may seem for one of the world’s great museums, which has a vast collection and receives and lends objects constantly, the BM has never had what any self-respecting theatre has always had: a get-in. A place where trucks can deliver and remove objects – often of great value and great fragility – securely.  Now they are finally building just such a get-in, a truck lift that will receive full-size vehicles and deliver them underground. Maximum capacity is three such trucks – two parked below ground, and one on top, behind the security gates. Compare that to the present arrangements, where trucks have to reverse from a narrow, busy street through the ceremonial gates into the museum’s public front courtyard, whereupon objects have to be manhandled up the steps and through the front hall. Even from that point on, things can go wrong, especially if the exhibition in question is to be in the former British Library Round Reading Room: on one embarrassing occasion a borrowed object turned out to be just slightly too big to get in.

So the new building has not just a big get-in but also industrial-scale lifts and ease of access to the new gallery. To show off, the opening show will have a full-length Viking longship from Norway, an object of a size not previously possible to display.

By the normally exuberant standards of Rogers Stirk Harbour, this is a subdued building, though it is highly prefabricated, hence the construction speed. You get the occasional Rogers-ish flash – a bright yellow steel beam, precast concrete floors set with circular glass lenses, some satisfyingly muscular steelwork – but in this august context they have turned the volume right down. The final public appearance of the building is yet to become apparent: its ‘veil’ of glazing, consisting of purpose-designed textured glass panels with clear borders, all at the time of writing stacked up in a warehouse waiting to be delivered and installed. And how they handle their areas of stonework will be interesting, from a practice not normally known for masonry.  So, no aesthetic judgements yet. But in operational terms, it is already clear what this means for the museum: the biggest step forward in its 260 years.