After a break from the 'eco' aspects of the project, this week we completed the installation of the rainwater harvesting tank. It stores 3000 litres of water and measures 2.5m x 2.5m, sitting in a hole 1m deep:
Rainwater is often overlooked in eco-house projects (especially urban ones). The whole rainwater system was bought as a kit from http://www.rainwaterharvesting.co.uk for just under £2,500. If it saves a few hundred pounds per year in water bills, then it will pay for itself in 5-8 years - which is a better return on investment than pretty much any other eco-feature. That doesn't take into account the labour cost of digging the hole, the disposal of the waste, or of the extra plumbing work involved. However, in our case we are also using the tank to feed the sprinkler system, so we can allocate some of these costs to meeting our fire safety requirement instead.
The system hasn't yet been commissioned though. Next week we're putting the scaffolding up at the rear and continuing with the EWI all the way up the back wall.