Too many flats and too few houses are being built in Wales's biggest cities, claims a leading planning expert.
Cardiff has seen the largest increase, with the number of new flats more than doubling in the past five years.
A decade ago, about 20% of new homes in Cardiff were flats; now it is nearly 70% in some developments.
Professor John Punter, of Cardiff University, said Wales's greenbelt land south of the M4 should be seen as the "natural repository of family housing".
"We're turning into a kind of 'apartment city'," said the professor of urban design and director of Design Commission Wales.
He said: "In effect, what is happening to our housing market is it is becoming all apartments.
"It does not cater for large sections of the community. Most apartments are one- and two-bedroom, so they are good for couples and people co-habiting and things like that.
"But as soon as people have children, they'd have to move out. And where would they go? Where would the new housing be?
"What's happening is that we're really not going to be building any new housing in Cardiff, the new family housing is going to be built up the valleys.
"That would be OK if the transport network linking Cardiff to the valleys was adequate to take that demand."
He called for a "more balanced allocation" of land for housing as a predominance of building on brownfield sites "almost invariably means apartments".
Professor Punter said greenbelt land should be looked at for houses
He said Swansea and Newport were "just beginning" the property-building boom Cardiff underwent from 2001.
One couple, Patrick and Claire Smith, are renting a flat in Cardiff Bay but are looking to buy, ideally a two- or three-bedroomed house with a back garden.
Mrs Smith said: "For us it's impossible at the moment. It's so expensive, over-inflated. There're very little affordable houses around.
"They don't make the little estates they used to have, it's just a lot of high-rise apartment blocks.
"We just feel we're banging our head against the wall trying to find something that's affordable."