Diary of an Eco-Builder

'It's un-green, but steel comes in handy when you're trying to be clever with interiors'

Tree House, a house that works like a tree, is a personal ecological fantasy slowly being made real in a quiet corner of Clapham, south London. It is driven by my belief that ecological design can be a "win-win" concept, delivering great results for us as well as for the planet, an idea that I hope is no longer fantastical in itself.

Tree House, a house that works like a tree, is a personal ecological fantasy slowly being made real in a quiet corner of Clapham, south London. It is driven by my belief that ecological design can be a "win-win" concept, delivering great results for us as well as for the planet, an idea that I hope is no longer fantastical in itself.

But we want more than this: we want to create sensational living spaces in which inside and outside merge, where the form, light, colour and textures of the organic world surround us. We want Tree House to be a stylish contemporary home, but we also want it to be magical.

In practice, ecological principles and our design ambitions have not perfectly converged. To be blunt, the occasional compromise has been necessary. This week our long-awaited super-structure started arriving on site, but the first components to be delivered were not made of Douglas fir nor engineered timber, but that most un-green of building materials - steel.

Although you can build six-storey buildings in wood, steel does come in handy when you're trying to be clever with your interiors. And our open-plan living space, extending through a glass wall into our courtyard garden, would not be possible without a steel architrave to keep the rest of the house up.

The metal-smelting industry is second only to the chemicals industry in the toxic emissions that it produces, and iron and steel production have the worst record for water pollution in the UK. The raw materials are mined, transported huge distances and then smelted in blast furnaces, consuming huge amount of energy at every step. And the problems aren't over when the steel gets on-site: such a good conductor can suck the heat out of a building unless every post and beam is highly insulated.

Nasty stuff. But because I wouldn't dream of leaving you with anything other than a warm eco-glow, let me persuade you that in the right hands, even steel can be an eco-product.

Enter Jonnie Rowlandson, metal-worker extraordinaire, based up the road in the Clapham North Arts Centre ( www.argonautdesign.co.uk). Over the last month, Jonnie has been scouring the scrap metal merchants of south London for sheets of rusting steel, old copper hot water cylinders and fragments of discarded brass. These salvaged materials have excellent eco-credentials, because whenever you buy something salvaged or second-hand you are keeping valuable resources in use and avoiding the extraction and manufacture of new resources.

With this gritty palette, Jonnie has made us the ultimate garden fence. Because we will have no fireplace in our ultra-low energy house, our garden will be our hearth and the back fence will be the focus of the hearth.

Jonnie has risen to this metaphor and created a fiery, organic, richly textured work of art, alive with the dense, complex colours of rusted and stressed metals. Like any fire, it will change over the day and over the seasons, a constant source of fascination and inspiration. In the middle of winter when the garden is quiet, this fire will be ablaze.

So although a well-hidden and thoroughly un-eco steel joist is critical to the engineering of our open-plan living space, it is rusty, salvaged steel that will transform our experience within it. Thanks to our local alchemist, the first expression of our magical ambitions has been successfully forged from the basest of metals.