A portico in east London has an unusual past and a brand new future as the entrance to a computer training centre. Emma Cullinan reports

"Everything is going to be alright", wrote artist Martin Creed in neon lettering on the entablature of what was left of a rather grand neoclassical building in London. And he was right. While Creed went on to win the Turner Prize two years later, in 2001, the remains of the building in the London Borough of Hackney a portico with two colonnades extending to each side - was turned into a computer training centre, by Brady Mallalieu Architects.

The current structure is the building's third incarnation and each time it has been used for the betterment of humanity (whether misguided or not). It was built as an orphanage in 1825, to designs by architects Golding and Inman, during the classical revival in architecture.

The central core was true to ancient Greece: a portico with its trabeated (post and beam) construction and simple Doric columns, with a chapel behind. This all spoke of Greek temples, but the building elaborated on that, with its colonnades to each side drawing the classical line out to wings that aped the form of the portico.

A typhoid epidemic in 1871 persuaded the orphanage authorities to relocate to the wilds of Watford and later, in 1881, the building was bought and revamped by the Salvation Army. The chapel was knocked down in favour of a 3,000-seater congress hall and accommodation was built on top of the colonnades.

The Sally Army vacated in 1969 and much of the building was demolished leaving, rather bizarrely, just the portico and colonnades. While similar structures have been built in the past as pure monuments - such as Napoleon's Arc de Triomphe in Paris and the Brandenburg gate in Berlin - the orphaned portico was an extraordinary relic of times past, sitting in the grounds of a girls' school, at the end of a small Victorian terraced street in one of London's most deprived areas.

As a door, it has become rather symbolic. This is the area where the London Olympics (another link with ancient Greece) will take place and this portico is proving to be a portal to both the past and future. Having housed orphans and later aided the path to God, this building is now being used to lift people out of unemployment, through computer training, under a UK government scheme called Excellence in Cities.

We've every right to be nervous about mixing classical and contemporary considering the blight of plastic "classic" porticoes added to the most inappropriate structures, but this building shows how the two can be linked well. The marriage has been conducted beautifully and simply, with a certain amount of wit.

It could also be argued that this form is true to original classicism. Ancient Greek buildings - the Parthenon being a case in point - usually comprised a portico on the end of a rectangular building. True, they were surrounded by columns; the difference in this contemporary example is that the lines on the new part run horizontally in the form of aluminium louvres.

The louvres are a device that welds the scale of new and old, although the juxtaposition between the two astonishingly different volumes is delightful. The scale of this huge spirit of ancient Greece, dropped into this typical Victorian London neighbourhood, is made all the more dramatic by the fact that it is on a small hill and because it doesn't have the usual elements through which we "read" the relative size of a building - that is, standard windows and doors.

So the architects took the decision to remove apparent windows and doors from the addition, covering most of the elevations with louvres (which were once hoped to be in timber but the budget didn't stretch that far).

It works because the louvres break a large volume into a delicate one, with the horizontal lines ushering the eye into the portico; not that you could miss it. Glass has become the standard material in additions to ancient buildings - because it treads so softly - but these louvres could point the way to other lightweight solutions.

Where glass would have been dramatic is in the staircase that ascends the reception area in the portico. This is the building's main orientation and circulation point and marks another great contrast.

Having entered the building from a smooth slice of classical architecture you emerge into a rough brick interior. Many Roman ruins, stripped of their render, have displayed their brick underbelly - as have period farmhouses and townhouses.

Conservationists may argue that original renders should be reapplied but the architects here wanted to give a sense of the structure.

That sense has also been preserved by maintaining a gap between the building and the new elements that have been slotted into it. These include the rendered stair structure which gives stomach-tickling views down over a balustrade from a great height (linking into the third floor of the new part). Wide oak window and door frames add warmth. "The budget was tight and this is where we decided to spend the money," says Angela Brady, who studied architecture in Ireland and went to London for a weekend two decades ago and stayed. She has set up in practice (and in life) with Robin Mallalieu and retains strong links with Ireland, through the RIAI (Royal Institute of the Architects of Ireland) and other organisations.

The computer rooms in the new part of the building feel like standard education rooms anywhere. "We even budgeted on the baffles," says Angela, indicating the rows of hanging strips of polystyrene. Actually, they look pretty good.

Brady Mallalieu are gaining a reputation for stylish buildings that further people's lives. They designed a 50-bed foyer in Dublin 8 - for 18-25 years old who can't afford a home of their own - and have recently completed a housing scheme for keyworkers (nurses, teachers and so on) near this portico. The main part of this housing scheme comprises a rhythmical row of five rectangular buildings sliced at a slant at each end. They present a friendly face, being in cedar shingle, self-coloured light render, brick and standing seam zinc finishes to the roofs. Deep glass clerestory windows float the roof over the rest of the structure and yellow uprights give these a sunny disposition. One resident is shopping for yellow bits and pieces to carry on the theme.

The designs of these low-budget projects show the benefits of keeping it simple and adding richness where the purse allows.