Diary of an Eco-Builder

How a simple pond project turned into a major feat of engineering

Last September, I began to tell the tale of our radical self-build in south London, a project that aims to use an exceptionally high ecological specification to deliver an equally high quality of life. Now, four months and a lot of mud-slinging later, life on our tiny Clapham building site is full of the joys of mid-winter: shivering in the shed, lingering in the caff, and bunking off as soon as the light begins to fail.

Last September, I began to tell the tale of our radical self-build in south London, a project that aims to use an exceptionally high ecological specification to deliver an equally high quality of life. Now, four months and a lot of mud-slinging later, life on our tiny Clapham building site is full of the joys of mid-winter: shivering in the shed, lingering in the caff, and bunking off as soon as the light begins to fail.

Fortunately, this behaviour is only typical of the owner (me), and not of site regulars Steve, George and Dennis, who are out in all conditions, determined to finish our ground works. Although the slab was laid before Christmas and now sits proudly on its nine-metre legs, there remains the small matter of our pond.

The main living space on the ground floor of Tree House has been designed as an inside-outside unity of three different ecological niches. Inside, we will enjoy fossil-fuel-free comforts, gazing through a wall of glass to the richly planted courtyard garden. Between the two, along the entire width of the house, a formal pond will provide a striking transition, itself home to a new community of many-legged beings.

When I dreamt up this ecological fantasy 18 months ago and scribbled it down for our architect, I had no idea that I was kick-starting a major piece of engineering. Because the pond sits immediately next to the house, it has to be integrated into the slab, and because it is relatively large, it has to be made of reinforced concrete and sit on piles of its own. On Thursday, after several days' hard labour preparing the shuttering and bending the (recycled) steel rods into place, a truck laden with (recycled aggregate) concrete arrived to complete the job.

All this will do more than improve the view and local biodiversity. It will also help to keep us cool - and keep power-hungry air-conditioners out of harm's way. As glass walls can turn a house into a greenhouse, we are pulling every trick in the sustainable design book to stay comfortable.

The design of the living/garden space is inspired by the classical Roman peristyle garden with its close integration of inside and outside and careful use of water and plants to keep the space cool. The citizens of Pompeii did not incorporate formal pools into their houses merely because they looked pretty, but because the evaporation of water draws heat out of the air - 600 calories per gram of water. Fountains are particularly good at encouraging evaporation,.

Trees and other plants perform a remarkably similar role: it's not just their shade that helps to keep us cool but the transpiration of water from their leaves. Trees sweat a lot of water, much more than they use for photosynthesis, nutrient transport and their own cooling. Evaporation from leaves draws water up from the roots through minute columns of water. With water and plants in the heart of our living space, Tree House will keep us cool in much the same way that urban trees keep whole cities cool.

If your house suffers from summer overheating, think first about shading, ventilation and fans. If you are still tempted to phone the air-conditioner company, ask about evaporative coolers. If the air is reasonably dry, you will get far more efficient cooling. Better still, try a visit to your local plant centre. It might make the difference between living with an ugly, noisy box and the delight of a lush interior garden.