Having made the occasional architectural pilgrimage to the building over the years and observed its flaky nadir as a seaside variety hall with gruesome municipal café attached, I'm cheering. Five years ago, despite being Grade 1 listed, it very nearly got sold to a pub chain. Now it is nearing the end of an £8m restoration programme. It's a surprisingly big place for something bearing the name "pavilion". No mere bandstand this, it contains a 1,000-seat theatre, art galleries and, soon, an upmarket restaurant, all right on the sea front. There may not be much to do on the windy, pebbly beach, but the views are great.

Perhaps it's safer to say that I expect to be cheering at some point soon. Although the re-opening date has already been put back from August Bank Holiday to October 15, even that date seems optimistic. The place is a total mess, swarming with builders, pneumatic drills hammering and spray guns hissing. The people fitting-out the first exhibition are treading on the toes of the people trying to get the restoration finished. There are two new discreet extensions at the western end which won't be finished for some months later anyway. The administrators ruefully admit exhaustion. As the guests arrive on October 15, they'll be pushing the builders out the back. But chaos like this is not unusual on a cultural building project. The point is, the big rescue is happening.

Why is the De la Warr Pavilion so important? It is a rare piece of architectural history, a landmark of early modernism in Britain. It would not have happened in this way, had not Hitler come to power in Germany in 1933. Immediately, an exodus began of artists frowned on by the Nazi regime. Erich Mendelsohn was an internationally-famous modernist architect who had sketched astonishingly original, Expressionist buildings while serving in the Kaiser's army in the First World War. Afterwards he rose to prominence in the Weimar Republic. He was among the first to leave when Hitler took over. Later he moved on to Palestine and then America but his first port of call was Britain. Here he went into partnership with the younger Serge Chermayeff, who had emigrated to Britain from Russia as a child in 1910.

Three months after Mendelsohn arrived, a competition was launched for an ambitious cultural centre on the Bexhill seafront, masterminded by the Earl de la Warr, local grandee and socialist mayor of this otherwise deeply conservative town. The competition brief had been written to encourage the new wave of modernists. There were 230 entries. Mendelsohn and Chermayeff entered, and won. It was one of those serendipitous moments in architecture when everything just slotted together. Political events elsewhere in the world meant that something radical and exotic happened to dozing Bexhill.

This was by no means the first modernist building in Britain but it was by an acknowledged pioneer of modern architecture, and the pedigree showed. Not only its appearance but its welded-steel structure was unparalleled this side of the Channel. The long, low white building, its rectilinear nature subverted by two glorious semi-circular stair towers north and south, was immediately recognised as a new departure for British architecture. And it was built in ten months flat. "Thank goodness we still open our gates a little now and then to foreigners," remarked the Architect's Journal.

Originally the plan was even more ambitious. Mendelsohn wanted a glazed two-storey pergola to enclose the lawns outside. Later he also added an outdoor swimming-pool and a pier, plus a hotel and cinema, to the design. None of these was built: with war approaching, the times were not propitious. But the survival off the building is key to an understanding of his work, since so much of his oeuvre in Germany, including buildings with some very similar features such as his Metal Workers' Union building in Berlin of 1929, have vanished.

The restoration has been carried out by architects John McAslan and Partners, who have been involved with the Pavilion Trust since 1989. Much of the Thirties interior detail was lost from the building in the 1960s and 1970s - such as all the fittings in the original library. Happily the glass cylinder-and-disc central light fitting rising through the main spiral stair survives, as does the clever dimple-coffered ceiling of the theatre. All the steel-framed glazing on the salty sea front had rusted so badly it had to be replaced, but the replacements are faithful, and designed to art conservation and security standards. Since all the original furniture had long vanished, a new De la Warr furniture range is being produced by award-winning designers Barber Osgerby. The circulation through the building is being improved, bringing the flat sun-deck roof back into use. Mendelsohn's more sheltered "sun parlour" on the next level down, which had been enclosed, has been reinstated as a spill-out space for the restaurant.

What emerges from all this is an arts centre where performance (from comedian Eddie Izzard and soprano Kiri Te Kanawa to the Ukulele Orchestra of Great Britain) and high-level contemporary visual arts exhibitions mix with seaside enjoyment. Art was not so much of a thing in the original - they thought people would rather sit in the library and read, or listen to improving lectures. Today, as the Pavilion's director Alan Haydon points out, Bexhill takes its place in a fast-reviving string of seaside towns where culture is on the agenda, from Margate round to Brighton. Even before the closure, he was getting 500,000 visits a year. Property prices have soared, young families are moving in, it's all change. Bexhill will never be either Sydney or Bilbao but it is clambering back onto the map.

http://www.dlwp.org.uk/ - De La Warr Pavilion
http://www.mcaslan.co.uk/ - John McAslan and Partners