• Put in solar thermal panels for hot water
• Install solar pv panels for electricity
• Fit a biomass boiler using wood pellets as fuel
• Put in underfloor heating
• Replace the double glazing with new high spec windows
• Fit a heat recovery venitlation unit
• Fit a water tank and pump to use rainwater from the roof
• Recycle greywater from the house
• Build a greenhouse
• Build a bike shed
• Turn the garden over to vegetable and fruit production

I would then feel this was a truly environmentally friendly house, ready for the uncertain future.