Whether or not a granny adjusts to life in the back garden or the basement of their child's house often depends on the granny, says Domhnall Casey, psychologist and psychoanalyst.

"Some grannies love having their grown up children and their grandchildren around them. Others may not be able to cope with being bombarded by marauding grandchildren. And they may begin to resent it if they feel they are being treated like a live-in babysitter."

And it has to be remembered that a granny is not just a granny but a mother-in-law too.

"Mothers-in-law often spell trouble. If a mother has not resolved the loss of her son, she might continue to interfere in his marriage, although people tend to get more mellow as they get older. Also a lot of negative stuff men have about their wives may be dumped on their mother-in-law."

When the arrangement works well, it is infinitely preferable to a nursing home, says Casey.

"In an ideal scenario if the elderly person has their own self contained area either in the main house or beside it, it can provide a real sense of safety combined with a degree of independence.

"You have to be respectful and caring of the older person and where they came from. If they have come from a big house, you have to be aware that they have taken a drop down in standards although some people as they get frailer are happy to not to have to take care of a large space."

Just like grannies, granny flats come in all different shapes and sizes. The more conventional semi-detached built-on or detached self-contained units "do not exist in massive volume", says Simon Ensor of Sherry FitzGerald.

"They account for 1 per cent or less of the market as not that many houses have the room to build on at the side or the back. More relevant and more common are two storey-over-garden level self-contained apartments or self-contained wings.

"Some properties are sold with the potential to build your own granny flat subject to planning permission."

How much a granny flat adds to the value of a property depends on the quality and size of the flat and the calibre of the main property.

Small converted garages with as little as 200 to 300 sq ft comprising of just a kitchenette, a shower room and tiny living area are not going to boost the value of the property significantly.

"Quite often they are so poky, you'd resist putting your granny in there," says Ensor. " In those situations it doesn't add a lot. You need at least 400 to 500 sq ft for a good one-bedroom self-contained unit. Corner sites with potential for a granny flat sell at a premium."

If the flat fits the above criteria, it can add anything from £20,000 to £150,000, (25,400 to 190,500) depending on the desirability rating of the house attached.

"On a good £700,000 (889,000) period home, it should add on £50,000 (63,500) but on a three-bedroom modern semi-detached house worth £250,000 (317,500) it is going to be less."

In the novel Bridget Jones's Diary by Helen Fielding, the heroine's father took refuge in "the Alconbury's dead granny's flat at the bottom of the garden" when her mother has a fling with a Portugese tour operator.

While a retreat from marital misconduct is not one of the more common uses, these days the granny flat is not just an annex for an elderly female relative.

In fact, in estate agent speak, the granny flat is no longer just a granny flat but a nanny flat or an au pair suite.

It can also be used as a den to banish noisy teenagers, a study or a guest suite.

Ed Dempsey of Jackson Stops says they seldom get the conventional granny flat on their books.

"I know someone who is looking for a granny flat. Some use them for staff and some are either officially or unofficially rented out. You get all sorts of conundrums."

Kirsten Bailey of Lisney says she more commonly receives calls from people asking for nanny flats.

"Very few ring up asking for a house with a granny flat. Mostly the requests will come from people relocating from England or other countries abroad where the nanny flat concept is very common. Very few Irish people would think of it."

"Mostly you are talking about self-contained basements so whether that is a granny flat or not depends on your definition.

Lisney currently has a property at 4 Ailesbury Drive in Donnybrook, Dublin 4, with a self contained area, own-door ground-floor wing with two bedrooms, one of which could be converted to a livingroom, and access to the garden. The asking price for the property is £1.35 million (1.71m).

The value of that self-contained unit is hard to quantify, says Denis Beare of Lisney.

"If you get the right person who needs a granny flat, it can put value on to the property and even if they do not need it for reason, it is further accommodation.

Generally, if you have two ordinary three or four-bedroom houses on a road and one has a granny flat - that has been done well - it can considerably enhance the value, but in houses like Ailesbury Drive, that value at over £1 million, it is difficult to quantify."

Having a separate entrance can also make it more saleable.

"If you are looking for somewhere to put your mother-in-law or staff it may help if it's a separate unit so they don't have to come into the main house all the time.", At the £1m-plus end of the market, it is more likely to be used to accommodate the live-in housekeeper, nurse, nanny or au pair. At the lower end, they are sometimes rented out, although a change-of-use permission should be sought from the local authorities - if you want to keep it above board.

How difficult it is to get permission, whether changing the use of the space or building from scratch, may depend on your neightbours. You might get it but it would not be not a foregone conclusion.

emorgan@irish-times.ie