Both kinds of buildings were originally designed to impress, on a civic scale, to handle relatively large numbers of people. Both are for obvious reasons usually to be found in town centres. And both have, in recent years, shown a marked tendency to become redundant. Which is fine. Because just as the decline in high-ceilinged high street bank branches proved to be a godsend for pizza parlours and coffee bars, so the availability and location of town halls and churches has seen all kinds of new arts venues take over.

Enlightened re-use is a pragmatic old game. Take churches. In London, everyone now knows how good the slightly over-restored Hawksmoor church of Christ Church, Spitalfields, is for music (unlike many others, it's an active church again as well). Easy to forget that it was nearly demolished in the 1960s, and was closed for years. Across town, its Baroque cousin by Thomas Archer, St. John's Smith Square, had been burnt out in the Blitz. Rescued with unusual sensitivity in the late 1960s, it has been a prime concert venue ever since.

More recently, the London Symphony Orchestra has settled into St. Luke's on Old Street, another church with a Hawksmoor connection that had been derelict for decades. But one of the best such changes of use is within a few steps of the Royal Court Theatre on Sloane Square: the 900-seat Cadogan Hall, home of the Royal Philharmonic Orchestra and much else besides. This eccentric Edwardian-Byzantine building was previously the First Church of Christ Scientist. Then it was briefly owned by Mohammed Al-Fayed, whose plans to turn this listed building into some kind of personal pleasure palace were flatly turned down. He sold it back to the Cadogan Estate, the big landowners round here, who then spent £7.5m lightly converting the church - which was in reasonable nick anyway -into an independent, acoustically excellent venue for all kinds of music, dance, celebrity talks, even puppetry - plus orchestral rehearsals and recording. Without the fanfare of public money being spent, Cadogan Hall opened very quietly last year but is steadily becoming a real force on the London music scene and is poised to launch an ambitious new programme later this year.

Cadogan Hall's conversion architects, Paul Davis and Partners, are now contenders for a top "Europa Nostra" conservation award this Friday (June 3) for their work, and I'm not surprised. It's the kind of award that is the exact opposite of the Stirling Prize, say, with its emphasis on icon buildings. Europa Nostra thus picks up the stealth buildings, the recycled architecture, that is otherwise off the radar. Cadogan Hall's auditorium is a wonderful space with a curved shallow vault of a roof and a deep horseshoe of an upper gallery..The church's original architect in 1908, one R.F. Chisholm, may have thought he was designing for Christian Scientists, but what he bequeathed the capital was a performance space of real quality.

This kind of thing happens more frequently than you might realise. The Soho Theatre, for instance, was previously a 1960s synagogue before being transformed by architects Paxton Locher a few years back. But enough of religious conversions: what about town halls and suchlike municipal architecture? Again, London has plenty of examples - mainly because, when all the boroughs were reorganised into the Greater London Council in 1965, smaller councils were absorbed into larger ones, leaving white-elephant buildings all over the place. Thus the well-regarded Battersea Arts Centre is in an 1898 town hall building by architect E.W. Mountford - better known for his later Central Criminal Court or Old Bailey. Rather like some of the converted churches, it faced demolition but was saved by a strong local campaign. Recently a similar campaign has rescued and refurbished the wonderfully grand Victorian/Edwardian Shoreditch Town Hall in fashionable Hoxton, east London. Built in two phases by Caesar A. Long, W.G Hunt and A.G Cross, good municipal architects all, it is now starting to be used - as so many of these old halls are - as orchestral rehearsal space.

Meanwhile in the media village of Crouch End in North London, plans for the differently magnificent, Dutch-modern-influenced 1935 Hornsey Town Hall by the young New Zealand architect R.H. Uren are such a hot topic that they even became a campaigning issue for candidates in the General Election of May 2005. I'll declare an interest as I'm listed as an unpaid adviser to the project: what's interesting about it - and this is borne out everywhere else as well - is how strong feelings are about such buildings. Having been centres of civic life, there is enormous loyalty and affection towards them. More loyalty for the buildings, indeed, than there ever is for the councils that own them.

In Hornsey's case, the asymmetrical L-shaped civic hall with its landmark tower - a place that was in living memory a venue for the Kinks and Queen, and still has most of its original 1930s internal fittings and furniture intact - may well become an art cinema as well as a live venue. Plus - once again - orchestral rehearsal space. There is also the question of what to do with the generous but under-used public square in front of it. The arguments over just how to achieve all this continue. It will cost around £10m.

Look elsewhere and you find plenty of municipal buildings already being re-used in this way. Birmingham's Gas Hall art gallery, part of the city's mighty Council House, was as its name implied previously the Gas Payment Office of 1910 (Stanton Williams did the conversion). In the same city, the Ikon Gallery (something of an ironic name, given its non-iconic premises) was previously a redundant school, made over by architects Levitt Bernstein with artist Tania Kovats. In High Wycombe, the old town hall is now an auditorium of the ambitious Wycombe Swan Theatre. The Camden Arts Centre in London, intelligently refurbished by architect Tony Fretton, is the erstwhile Hampstead Central Library. The McLellan Gallery in Glasgow began life as a private art gallery in the 1850s, then became a department store, then exhibition halls, and finally reverted to being a gallery again for the city's turning-point Year of Culture in 1990.

So the continuing debate about iconography or modesty in architecture - an eternal argument, since there always have been and always will be countless examples of both - rather tends to overlook the blindingly obvious fact of architecture that's already here, some of it pretty fancy stuff. You can build your new cultural buildings in any style you want, and hope that they don't leak, rust, or have bits start to fall off them. Or you can look at the rich stock of disused and under-used buildings already out there in prime locations and think - hmmm. I wonder if they're trying to tell us something?