By Hugh Pym
Business correspondent, BBC News
Today's buy-to-let mortgage figures from the Council of Mortgage Lenders suggest a booming market.
Beetham Tower is said to be Europe's tallest residential building
That was the case for much of last year but looking forward, things are starting to look rather more shaky.
Finding tenants is apparently no more difficult than before in most of our towns and cities.
For investors hoping to sell their properties, however, there could be a struggle ahead as a glut of newly built flats drives down rents and prices.
Apartment blocks sprang up as developers tried to meet the demand from buy-to-let.
Back in 1997, just 16% of new housing projects were accounted for by flats and maisonettes.
A decade later, last year, that figure had shot up to 46% of all new residential building work.
One such apartment development is Beetham Tower in Manchester.
Billed as a landmark development, it has been marketed as the UK's highest living space.
It accommodates a five-star hotel and cocktail bar as well as 219 flats.
Even before the building was completed two years ago, all the flats were rapidly snapped up.
Many were purchased by investors.
Some might have hoped for a quick resale and capital gain.
But the BBC understands that as many as a quarter of the flats at Beetham Tower are now on the market.
Some have had prices reduced as owners strive for sales.
Local estate agents, who did not wish to be named, suggested that the development was finished and sold at the top of the market.
The view from Beetham Tower
Capital values since then have struggled to keep up.
Tenants can be found but rents do not necessarily cover investors' mortgages.
The Beetham Tower experience is mirrored at other developments in Manchester.
Rebecca Williams, sales manager at Knight Frank's Manchester office, said: "I think that the speculative investor isn't quite as frequent as before."
"We've gone through such a boom period in the last 15 years where we have seen property prices increase to such a large degree - it reaches a slight plateau," she added.
Manchester is not alone.
Leeds, Nottingham and Birmingham have all experienced breakneck growth in apartment block construction.
When it comes to sales, as opposed to lettings, we may be seeing an onset of the "buy-to-let blues".