The humble one-bedroom apartment has come into its own - at least as far as the residential letting market is concerned. There's a shortage - and even at a time of year when other parts of the letting market can be sluggish, the one-beds keep moving.

Investors have traditionally tended to veer in the direction of the two-bedroom apartment in the hope of guaranteed appreciation, in the belief that they are easier to sell on. There is also a conviction that from a letting point of view, they are more attractive, given that you can pack more people inside. But not necessarily. Many people - especially older people and refugees from marriage break-ups - want to live alone.

And affordability matters in all cases. The apartment market is dominated by young professionals who have only so much to allocate on rent. They may be saving to buy a home of their own or intending to move on elsewhere. Couples prefer the one-bedroom to the two-bedroom apartment. They are increasingly being forced into the rental sector rather than automatically buying a house together. Hooke and MacDonald reports that the lower to middle market - the £500 to £1,000 per month bracket - accounts for 80 per cent of lettings, and is comprised mainly of one and two-bedroom apartments, predominantly in the city centre. They report a "significant shortage" of one-bedroom apartments for letting in Dublin, partly because tenants tend to stay longer. "They are normally occupied by single people or couples who are popular with property owners - hence the rent increases when a lease expires are less than the market averages," says Carina Warner. This helps to keep the tenants in situ.

One of the more sought-after areas at present, says Ms Warner, is Smithfield Village, close to the city centre, with one-beds costing from £500 to £550. Some tenants in this area work in the industrial estates of west Dublin, and commute by public transport. And tenants here don't have to incur the expense of a car if living in the city. Rents are as low as £450 for Sackville Court, in Blessington Street, while Custom Hall, in Lower Gardiner Street is letting from £475. But for £800 a month, Hooke and MacDonald will rent you a one-bed for £800 in The Printworks, in Temple Bar.

Carina Warner says investors were starved of two-bedroom apartments in the early 1990s, but "the balance has since swung towards the construction of mainly two-bedroom units in most new developments." Provincial buyers are particularly attracted by the two-bed units. Prices in the one-bedroom rental sector have risen faster than in the two bedroom sector, says Denis Kavanagh, of Gunne. He says that in the Milltown area, for instance, rents for one-beds have risen from £450 to £600 over the past couple of years, but proportionately, the rent for a two-bedroom unit has only gone up from £650 to £750.

Denis Kavanagh finds that one-bed apartments are walking off his books. "Investors very rarely buy one-beds in my experience. They have this idea that a two-bed is going to be easier to resell. I'm forever saying to investors to buy one-beds. If I had a building site, I'd fill it with one-bedroom units."

He also cites the example of the Waterside development in Charlotte Quay. When initially launched a few years ago, the one beds went for £380 to £400 a month, whereas the going rate now is £500 to £600.

The one-beds are an attractive option for the young couple who are not necessarily saving to buy a house, but are not sure of their own status. "Buying a house is a huge commitment for them," says Mr Kavanagh. "They don't want to move to a two or three-bedroom semi in west Dublin. They could afford the mortgage perhaps, but they aren't ready to make that decision."

Couples tend to stay in the one-bedroom apartments. "The rent will go up by a percentage minimum of 5 per cent but the property keeps in tune with the market. The tenant doesn't have the fuss of leaving and won't get a better rent in all likelihood."

The tendency of tenants of one-bedroom apartments to renew leases is also confirmed by Katherina Cahill, of Home Locators. "Overall, the one-bedroom apartment is hard to find and the tenants end up renewing lease agreements. The turnover is low."

One of the problems is that many of the new developments in recent years had only a small number of one-bedroom units. These include some of the top addresses such as the Sweepstakes and Ballsbridge Woods, which have mostly two, three and four-bedroom units. `The one-bedroom units are popular to rent and hard to get. The rent goes up at a higher rate of percentage than in any other type of property we're letting out," says Katherina Cahill. The most popular areas are in the city centre and the south inner city areas as well as the north city quay areas.

A one-bed in Garville Court, Rathgar, costs £650 a month; a one-bed in College Gate, Townsend St, costs £600 a month, while a ground-floor one-bed unit in Rainsford Avenue, off Thomas Street, is for rent at £520 a month.

Even out-of-town demand for one-bedroom units continues. Lisneys has recently opened a residential letting division in the Dun Laoghaire branch and Geraldine McMenamin says demand in the south suburbs for the one-bed unit is huge. "One of the problems is that wages haven't risen as fast as rents and that is why one-bedroom apartments are attractive. But if rents went too high, the demand would lessen," she says.

Those wanting to live alone are also in the market for the one-bedroom apartment, but this sort of tenant is being squeezed somewhat by the present situation. "People prefer one-beds if they don't want to share," says Geraldine McMenamin. "If they rent a two-bedroom apartment, they have to find someone to live there with them and most people over 25 don't want to do this," she says.

While rents in the south suburban belt average £500 to £600 for a one-bed flat, Lisney recently let a studio apartment in Monkstown for £400. But such properties are hard to find. Indeed, although the term "bedsitter" is used less and less, there remains a strong market for self-contained, compact accommodation for one person.

Katherina Cahill also says the lone tenant is having problems finding accommodation.

"I would sympathise with young single people living on their own," she says. "A house share offers a better standard of accommodation for the single person with a limited income but it's not what everyone wants."

Indeed, older people generally don't want to share. Some people have left the family home following a marriage break-up and want independent accommodation of a certain standard, but at an affordable rent.