Forget the leaky, lean-to of old - today's version is an very energy-efficient space designed for everyday living, year round, and not just for ornamental cane furniture and potted plants. That's particularly the case if you are talking about the bespoke glass buildings/ garden rooms/ orangeries which leading UK company Marston & Langinger specialises in.

And notice we still haven't used the word 'conservatory'.

"We rarely describe them as such because traditional conservatories have such a bad image, " says Sharon O'Callaghan, Irish architect and designer with the company. "The timber and glass structures we build work as light-filled rooms that become an integral, informal part of the home. They pour light into the house and there is a beauty and pleasure to them that draws people to their warmth. And modern glass extensions are warm - one of the things about them is that they are built for yearround use, for dining and relaxing in, and not just for during summer."

As we chat in one of the garden rooms Sharon has designed for a Dublin client, the abundance of light is just one of the feelgood factors of the space.

This garden room also functions as the obligatory 'family' room. It is open from the kitchen, glazed on three sides, with French doors to a sheltered courtyard garden and a glass ceiling that soars to a double-height architectural feature in its own right.

With so much glass, you normally would succumb to the shivers if using such a space as a winter dining room, but the double-glazing used is argon-gas-filled and incorporates a low-emmisivity (Low-E) to banish condensation and allow year-round use, explains O'Callaghan.

All of the double-glazing has UV controlling glass, reducing the transmission of ultra-violet light.

In green terms, glass undoubtedly lasts for a long time, but is recyclable.

Marston & Langinger also uses seasoned timbers from sustainable sources, while its comprehensive range of paints offer water-based, solvent-free and non-toxic formulations, which it claims have the longest life of any equivalent brand on the market.

For anyone considering extending their living accommodation by redeeming that gloomy space at the rear, enlisting the services of the professionals will be money well spent, says O'Callaghan.

The other route of course is to find an everyday architect and then hire contractors to do the work. Hiring a specialist garden-room company is more straightforward, she says. And even the most awkward of spaces can be transformed.

"If you are designing something bespoke, you can overcome any problems. We work with a lot of older buildings and come up with a sympathetic take on the modern glass extension.

"In recent years, there has been a fashion for adding on simple glass boxes, which in theory look very contemporary, but people can't live like that.

"Our garden rooms are an extension of the living and dining space in a house. Off the kitchen is the most typical usage - that's what most Irish clients want and opening directly as opposed to through glazed double doors.

It also makes for a better relationship to the garden or patio.

North-facing to the rear is not a problem - it's about introducing extra light more so than direct sunshine.

"The over-riding factor for people is to make better use of the space to the rear, particularly in the average townhouse where the back of the house juts out leaving a side alley."

All of that extending doesn't come cheap however - you may end up extending the mortgage to pay for it, particularly when the bespoke glass building in question ticks most of the eco-friendly boxes.

For a Marston & Langinger custom-made glass room, a rough price guide for an average 6x4 metre extension starts from about Euro75,000.

That's for the timber and glass, and the construction of same - but doesn't include digging and preparation of the foundations.

You will need to hire a builder separately, which could add another Euro20,000 to the bill.

For the final project, you could be looking at between Euro80,000 - Euro100,000 says O'Callaghan.

For further information contact 1800 635 081;

www. marston-and-langinger. com