The Yorkshire Sculpture Park is true to its name: a park with sculpture, in Yorkshire. It sprawls over 500 arcadian acres of the landscaped 18th century grounds of Bretton Hall outside Wakefield, dotted with changing exhibitions of the work of Henry Moore, Anthony Caro, Eduardo Chillida, Barbara Hepworth, Anthony Gormley and others. There are rival open-air galleries - the Cass Foundation at Goodwood in Surrey, Roche Court outside Salisbury, for instance - plus there are places such as Kielder Forest in Northumbria where contemporary artists are let loose among the trees. But the YSP is the one that invests most heavily in new architecture. It has now finished a £13m, three-year building programme. It's as much about the indoors as the outdoors, these days.

The park has had indoor display space for a while, tucked into existing estate outbuildings. Moving on from that, the first big expansion back in 2001 was the creation of the Longside complex - a set of ingeniously-converted barns high on the valley side, designed by Bauman Lyons architects, where the Arts Council's national collection of modern sculpture is now housed. Then things accelerated. After Longside came the Park's new entrance and visitor centre, an ambitious building by architects Feilden Clegg Bradley. These now lead on to the so-called "Underground Gallery", which opens on May 13. This is big and clever and at a stroke transforms the YSP into an altogether different proposition.

The first show is a William Turnbull retrospective, the biggest that this key figure in modern British sculpture has enjoyed since the Tate in 1973. The new spaces mean that Turnbull's more delicate work - including paintings and drawings - can be displayed in climate-controlled conditions alongside quite sizeable indoor pieces, plus the big outdoor works as well. You will be able to move from the small to the large, the fragile to the super-tough, inside to outside. After Turnbull this summer, they've got James Turrell coming in with his experiments in light and space for next winter's show. It's now a genuinely year-round venue. No-one can accuse the YSP, under its founding director Peter Murray, of failing to aim high.

I wasn't sure about the idea of an underground gallery, but when you see it you find that this 50 metre long, £3.5m building is in reality a terrace. It is set like an 18th century "ha-ha" into a slope - the estate's "bothy garden" - rather than being troglodytic. What you get is a set of three big rooms - the central one twice the size of the ones to either side. There is very little architecture to see from the outside apart from a long glass wall on the terrace side, but that does not matter since to get there you have already come through the highly visible introductory buildings by the same architects. Inside, it is just that triptych of simple, tall, white-cube spaces opening off the long stone-floored lobby via full-height timber sliding walls. Daylight floods down into the back of the galleries via a full-length horizontal skylight. That is all: these are sophisticated spaces but they are as pared down as possible. There is almost nothing here to interfere with the manipulation of space and light. No distractions.

Totting up its combined square footage, the YSP reckons it now has as much indoor exhibition space as the rather better-known Baltic gallery in Gateshead. It is probably inevitable that the park's marketeers should have come up with the arch phrase "A view with a room" to describe the new character of the place. But however you describe it, the development of the YSP to this point is a formidable achievement. A great country estate which had been split up has been reassembled. The new buildings have been judiciously dropped into place. What started off in 1977 as an idea for an outdoor sculpture exhibition has evolved into something richer and more complex. Almost by stealth, it has become a nationally important contemporary art gallery.

www.ysp.co.uk - modern art in arcadia.
www.feildenclegg.com - catch up with its architects.