Diary Of An Eco-Builder

Tree house is nearly finished, I'm feeling optimistic and our 'SAP' is running high

The end is nigh. Or so it seems, as the timber cladding on the front of Tree House is finally screwed in place. For the first time, it is possible to take in the full arboreal form of the house: the vertiginous cedar-clad trunk, the branching Douglas fir of the roof structure, and the muscular oak-framed windows that move up the face of the house as our tree's bare limbs rise to frame the cold winter sky. This project has not been without its problems, but in time they will all become shadows in the bright light of a creative ambition that has been realised.

There's still plenty to do before we move in, but tree and house feel increasingly like equal partners: two striking timber structures, exposed to the elements, drawing their energy from the sun and offering protection to the creatures that seek refuge under their canopies. Of course, we still don't know how the house will perform in practice, especially whether or not we shall achieve our "zero carbon" goal. But this week I did have the chance to get our figures checked out by the experts at NHER (National Home Energy Rating, www.nher.co.uk). To my relief, its results were remarkably similar to the sums I did long ago.

From June 2007 a home energy rating will be required in every home-seller's information pack. This rating will be calculated using a simplified version of the Standard Assessment Procedure (SAP) which is used to satisfy the energy-related building regulations. The SAP is currently changing its focus to carbon emissions, so from next year a new house ought to emit 20 per cent less carbon dioxide than a house built to today's regulations (an average of 2.6 tonnes per year instead of 3.2 tonnes).

One way to reduce carbon emissions is to install on-site renewable energy. NHER is using Tree House as an example of how an efficient and airtight building fabric can be combined with renewable energy to achieve a very high SAP rating.

The new SAP runs from 1 (hopeless) to 100 (no energy bills), with anything over 100 reflecting a net income from energy generation. Although Tree House came out at a disappointing (if excellent) 94, this reflects an assumption that the price we get for electricity we sell to the grid will be less than half the price of what we buy back. In reality, most current deals are far better than this. On purely energy terms, we should come out with a 7 per cent surplus.

If you want to get a simple energy rating for your home, there is a DIY calculator on the Energy Saving Trust website ( www.est.org.uk). If you want to pay someone to do the bells and whistles for you, contact NHER on 01908 672787. It's a great way to get a really detailed picture of your scope for saving energy.

Ask me in a year's time how all these calculations fare in practice. Right now, I am hopeful, inspired by the emergence of a building that is no less exciting than our extremely well-thumbed architectural drawings.

If I am moved by the flowering of the house, this is because in the moment of creation, another point in the future is necessarily defined: the moment of return to nothingness. This is not an unhappy thought ("everlasting life" is, for me, the ultimate oxymoron) but it is a sobering one. I noticed this week that the Green Party has a new leaflet illustrating "The British Isle" post Arctic meltdown: a shrivelled, emaciated remnant of our green and pleasant land. Clapham is not on it.

But I have never owned enough hats to sustain a millenarian outlook. The end is not nigh. It may take all our creativity to keep it that way but 'tis the season to be hopeful and I plan to keep my optimism alive for at least the next 50 years.

Will Anderson's complete 'Diary of an Eco-Builder' will be published by Green Books in spring 2006 ( www.treehouseclapham.org.uk)