Earlier this month, Capital Shopping Centres took the brave step of announcing a revaluation of its properties along with its interim results. The move has caused much comment within the property sector, some from competitors who passed off the measure as a publicity stunt and some from rival companies which say they are considering the same.

But to an outsider, the move raises the question that if net asset values are the single most important measure of a property company's performance, why are shareholders content with valuations that occur only once every 12 months?

Within the industry, interim valuations may appear novel. But to outsiders, their novelty value only illustrates the opacity that characterises much of the sector and sets it apart from other asset classes.

Of what use, in a volatile market, is a net asset value that is 11 months out of date? And what are we to make of the discounts or premiums to net asset value at which property company shares are trading? Are they still meaningful measures when a property company's official NAV calculation is 12 months out of date?

Instead of wondering whether CSC's move is wise, should investors be asking why it is not the norm?

Property companies say they are deterred from more frequent valuations by the cost. Moreover, they say, investors have never requested such a thing.

Ian Watters, director at MEPC, the UK's third largest property company, says his annual valuation fee is roughly £500,000 sterling and that would grow significantly if done more frequently. "Property values don't go up that quickly," he says. Indeed, it was only two years ago that the Accounting Standards Board even required UK companies to provide an interim balance sheet, first raising the issue of interim property valuations.

"And it's not just money," Mr Watters says. "Valuations take up management time and that's time they're not spending adding value."

ODDLY enough, these arguments have never seemed to deter the managers of property unit trusts from revaluing quarterly. Nigel O'Sullivan, partner at investment consultants Bacon & Woodrow, says he will not recommend managers who do not provide sufficient regular information on the properties they hold. Property unit trusts revalue monthly because they are open-ended vehicles and they need to tell investors the prices at which they may purchase and redeem units.

But Mr O'Sullivan points out that even vehicles that do not need to publish such data, such as property limited partnerships, generally do so at least quarterly.

Fierce competition among chartered surveyors has forced valuation fees down sharply in recent years. David Martin, partner at chartered surveyors Hillier Parker, estimates they are no more than one-half to one-third the level of five years ago. According to Rupert DeBarr, partner at Drivers Jonas, it is not unheard of for property companies quietly to approach their surveyors in between valuation periods. "During the recession, we were giving advice to plc's on a half-yearly basis," he says.

The real problem, he argues, is cultural. "The property industry has been slow to take up all sorts of modern developments and this is one of them." Some property company executives are dismissive of the chartered surveyors' arguments, noting that more frequent valuations mean more money in the surveyors' pockets.

ALSO, in a recession, more frequent valuations pose new risks. One property company chairman says he had considered, but rejected, the idea of twice-yearly valuations for that reason alone. With annual valuations, borrowers have more time to reverse their fortunes, perhaps selling a property if necessary.

But Peter Farnfield, director of Pricoa Property Investment Management, whose funds are valued quarterly and monthly, points out that for stock exchange-listed property companies, valuers are not the only source of information. "These property companies are well covered by the (sell-side) research analysts," he says. "They take over the valuation process."

Michael Mallinson, author of a landmark study on valuations, says valuations more frequent than every six months add little to investors' knowledge. Instead, significant developments such as receipt of planning permission on a key property should be reported to the market along with some estimate of the impact on NAV. That way, he argues, the markets can judge a property company's value for themselves.