KEEP OFF THE GRASS - IT'S THE ROOF

The Tate Gallery wouldn't have happened without the sugar cube, and the Horniman Museum in Forest Hill, south-east London, owes its existence to the tea packet. In 1898, the tea tycoon and philanthropist Frederick Horniman commissioned a building to house the eclectic souvenirs of his extensive travels: natural history, arts and crafts, and assorted curios. Charles HarrisonTownsend's Art Nouveau masterpiece, graced by a supremely elegant clocktower, opened to the public in 1901. Nikolaus Pevsner called it "one of the boldest public buildings of its date in Britain".

Now the Horniman Museum has been joined by an ecological building. The Centre for Understanding the Environment is made from timber and recycled newspaper - with a grass and wild-flower roof. It does look a little incongruous alongside Townsend's Victorian creation - English Heritage was predictably full of trepidation - but aesthetically the juxtaposition of styles works astonishingly well.

CUE is a light, open, airy space: a little oasis off the South Circular Road. Its relationship with the Horniman's 16-acre garden is plain to see: everything inside is connected with everything outside; interior and exterior are separated only by glass walls and huge picture windows. Fred Horniman would surely have approved, though he lived decades before "green" became a concept.

CUE was commissioned by Dr Elizabeth Goodhew, until recently a curator at the Horniman, who wanted to raise the awareness of ecology through a building with an environmental message in its design. She had been impressed by the nearby London Wildlife Garden Centre, which was designed by Jonathan Hines of Architype, a practice with a particular skill in green design. Hines agreed to complete the CUE building in seven weeks.

"We called it our longest and slowest project ever," he says. The roof is still growing - and it always will be. Built from sustainable timber, CUE is insulated with recycled newspapers and finished with non-toxic organic paint. The roof is irrigated by pond water raised by a pump operated by solar power. When this water evaporates, the building is cooled, an effect which will increase five-fold when the meadow on the roof grows to a height of about 20 cm. (Here there is a setback, as crows have attacked a section of the roof that now needs to regrow.)

CUE's impressive array of ecological features includes an innovative structure of pre-fabricated hollow timber beams and columns that create a system of natural passive ventilation, reed beds to recycle water, and photovoltaic panels to generate electricity. A computer controls the building's use of energy and its air and water movements.

Hines is justifiably proud of his pioneering work. "In our modern, synthetic world, our sense of smell is stultified by a constant battery of pollutants; our sense of touch is diminished by smooth artificial materials, and our sense of vision is over-stimulated by a vast range of fluorescent and synthetic colours," he says. "CUE aims to demonstrate a human and ecologically sound alternative in which, for example, organic paints made from natural oil can enhance and inform our sense of smell; natural materials with a variety of textures and finishes stimulate our sense of touch; and colours, based on natural pigments, create a natural harmony with our sense of sight."

From the outset, CUE was planned as an environmentally friendly, futuristic building. But what makes it especially fascinating is the way Hines has used green materials in such a creative way. The CUE's smell - of fresh air and wood - and its sense of optimism and well-being are remarkable. It is truly a building of the future, and quite possibly a prototype for many more.

!CUE at the Horniman Museum: 100 London Road, SE23 (0181 699 1872). CUE can be seen today, for nothing, as part of "Open House", the annual weekend celebrating and enabling access to hundreds of London's buildings; call 0891 600061 for more details.