With the opening of its new extension, Tate Modern has greatly increased in size. It was already pretty big, as converted power stations tend to be, but the addition of an eleven storey building makes even more space inside for art.
Tate Modern now consists of three parts: the Boiler House and Turbine Hall (which you already know well) and the brand new Switch House added on the far side. This has been built on the site of an electricity substation closed ten years ago, immediately above three decommissioned oil tanks. The original design for a glass pyramid was replaced by something similar in brick, to match the power station, with twisted layers decreasing in size as they rise. It's a striking design, and could look wildly out of place, except that this plaudit has already been taken by the cluster of luxury apartments squeezed in close by.
First things first, how do you get inside? It turns out there are four ways, and from outside just the one. The Switch House's main entrance is round the back of Tate Modern, at the foot of the brick ziggurat, immediately between the signs that say BAR and SHOP. There is another larger entrance here, but this leads into the original building, specifically the first floor bridge across the Turbine Hall, and then you have to cut back through the gift shop to get where you want to be. Alternatively you can get in from the Turbine Hall, down at Level 0, which leads directly into The Tanks where you'll find lifts and a staircase leading up. But the most thrilling way to enter is from the Boiler House via the fourth floor overbridge, a brand new connection high above the chasm of the Turbine Hall. If you want a vertiginous view down, feel free to wave your camera over the edge, but if you don't have a head for heights stick closely to the centre line and walk fast.
0: The Switch House's basement is called The Tanks, and is probably where you'll start. These circular subterranean spaces opened temporarily in the summer of 2012, so you might already have been, but then had to be closed to allow building works to take place on top. There are three tanks in all, each generally bigger than you think they're going to be when you walk through the door. This level is given over to interactive art and video installations, plus performances of various kinds which this weekend include people being ordered about as human sculptures. Or lie down on the comfy red cushions in the dark to watch a group of teenage Thais on a dozen screens, or step through a blue chamber and watch the lights change as you pass by. And don't miss the tiny gallery at the foot of the stairs with a display of homewares and furniture by the designer Jasper Morrison, which is much more interesting than its 'Thingness' title might suggest. Going up.
1: Hmm, the only things open to the public on the ground floor are the gift shop, the terrace bar and a lift lobby. There must be a lot of space hidden away round the back. Going up.
2: And here goes with the stuff. There are no paintings in the Switch House, only stuff, and the largest gallery on the second floor is replete with it. A lot of the stuff in here is geometric, including a lush pink cube with curved edges, and that pile of bricks the Tate famously blew its money on in the 1970s. I had to wonder whether twelve metres of blue and white cloth was really worth its place high on a sparse wall, and would have enjoyed the bubble fountain more if a group of schoolchildren hadn't taken up position all around. One work that did intrigue was a long stripof bunting being held by two strangers, apparently for hours, enforcing a separated connection between the two... until I looked round and saw that one of the participants had been silently replaced. Going up.
3: Of the Switch House floors with art, and there are only three, this one held my attention least. That may have been because the Brazilian exhibit was without its macaws - they're being kept away just for this opening weekend because the visitor numbers are expected to be so high. But I did love The Crystal Quilt, a video of a gathering of ladies in Minnesota coming together to discuss growing older, sat at tables temporarily arranged to match the tapestry of the title. And if two things should be obvious by the time you've got this far, they're that the Tate now gives greater than usual prominence to works by women and to works from overseas... indeed now a proper balance, which is long overdue. Going up.
4: By now the walls of the pyramid have shrunk a little so there are only two galleries on the fourth floor, plus a large sloping lobby that doubles as a potential performance space. One is devoted to the works of Louise Bourgeois, whose towers and giant spider dominated Tate Modern's Turbine Hall when it first opened, and one of her smaller arachnids stalks the space. The other includes works that reflect on cities, including a map of Beirut you can walk on, and what I thought at first was a town of sandcastles but on closer inspection turned out to be made of couscous. There really are so many opportunities throughout the Switch House to exclaim "well that's modern art for you!"
I should at this point mention thestairs. A grand staircase sweeps up the centre of the building from the basement to the fourth floor, curving and twisting irregularly as it goes. This provides more than just a connection, it's a promenade for visitors, a route to help you feel at one with the building. Or there are lifts, one set just running from zero to four, while the bank opposite rises all the way to the top. And everybody wants to go to the top, because that's where the viewing gallery is, so this weekend the 0-10 lifts have been hugely oversubscribed. Long queues have built up by The Tanks, which are temporarily the only point of access, as humanity attempts to squeeze in and ascend. It won't always be this bad, but there is another way to reach floor 10 which is to take the smaller unsigned staircase from floor 4. You'll need to be fit because there are 242 steps, indeed the entire staircase from basement to summit has 408! But you will arrive smugly on the roof, plus you'll get to peer at all the other levels inbetween. Going up.
5: There's no further public art to see, although this level is devoted to dialogue about art and will open up to a programme of events from September. Going up. 6: This level has two rooms, both being used (and to be used) as event spaces. Going up. 7: This level is staff only, so the lift lobby is an oasis of calm on your upward ascent. Going up. 8: This level houses the Members Room, a counterpoint to the Members Bar at the top of the Boiler House. Going up. 9: And here's the restaurant, because any modern art gallery lives or dies by its foodie offering these days. This one has a particularly large plate glass window allowing those ascending the stairs to peer in at the immaculately laid tables, so try not to get seated too close to it. Going up.
10: Finally, be it by lift or by stairs, you'll reach the top floor. Brilliantly this is a viewing gallery with terraces along all four sides of the building allowing 360° views. From the front that's across the multifarious roofs of Tate Modern towards the Thames, with the brick chimney and St Paul's Cathedral the two dominant features. To the side that's across towards the Shard, whose viewing gallery may be considerably higher but also costs infinitely more to visit, so I can see the Switch House becoming a vantage point of choice for many Londoners. And round the back are the residential towers of Neo Bankside, whose exo-skeleton design saw them shortlisted for the Stirling Prize, but whose residents now find themselves being stared at by artlovers wielding zoomable cameras. I watched one couple in their triangular glass living room sitting down for lunch, and another resident opening up a copy of the Financial Times with a cup of coffee. Perhaps you had to be an exhibitionist to want to live so openly in the first place, but many may now be regretting their luxury purchase and its lost Thames view.
So that's the Switch House, a massive undertaking and I'd say a great success. Tate Modern is already London's most popular visitor attraction and now has space to breathe and space to grow. A journey round the new building genuinely feels like an adventure, the irregular nature of the rooms and passageways inviting personal discovery. It makes the 'old' Boiler House look positively predictable by comparison, but that still has more to see, and might even be rather quieter now this overspill has opened. Whatever, it's now a lot easier to spend much more of your day at Tate Modern, or to dip back in repeatedly to a building you'll learn to know well. [40 photos]