When is an eco house not an eco house? When Prince Charles builds one. Or so the boffins at the BRE would have us believe.

The uneasy alliance between Britain’s green building establishment and the Prince threatens to be blown apart by the decision to allow the Princes Foundation to build its version of an eco house at the BRE’s demonstration site in Watford. Currently, it’s home to hi-tech eco houses, notably the Kingspan Lighthouse and Stewart Milne’s Sigma House. But the Prince’s eco house will be decidedly low tech. It will almost certainly eschew modern methods of construction, will probably eschew all plastics and, horror or horrors, will cock a snook at the eco builders No 1 bugbear, airtightness.

The Princes Foundation has been remarkably successful in promoting the Poundbury vision across the UK. It’s become a significant player in the world of master planning — essentially the art of playing SIM City (aka God) with yet-to-be-built settlements, which is perhaps a natural outlet for our future king and his courtiers. Though Poundbury itself is ultra-traditional, the lessons learned from its placemaking seem to be applicable across all kinds of sites and crucially all manner of architectural styles. Thus the work of the Foundation has been accepted — welcomed even — by big hitters in this arena, the DCLG, the architectural establishment and influential bodies like the BRE. They seem keen to court the Foundation and often work closely with them on projects.

But beavering away in the background is the Prince’s other agenda, his pathological hatred of modern architecture. By granting him the opportunity to build his trad version of an eco house at their show site in Watford, the BRE have handed the Prince the opportunity to put his tanks quite literally onto their lawn.

Jay Skandamoorthy, who is the BRE’s director of Enterprise and Innovation is quoted as saying that the Prince would be taking “a different approach.” Too bloody right he will. He added “The Code for Sustainable Homes will not be a driver for the prince, but we expect the scheme to be rated at Level 3 or 4.” Is that catty or what? Anyone whose anything in the mainstream Green Building movement knows that if it ain’t Level Six, it’s not worth the candle. Bill Dunster, builder of Bedzed, spiritual home of zero carbon homebuilding, thinks we should now all be aiming for Level 7. In this context, Level 3 or 4 is a slap in the face and, worse, will not be permissible after 2016 when all new homes will have to meet Level 6.

By granting him the opportunity to build his trad version of an eco house at their show site in Watford, the BRE have handed the Prince the opportunity to put his tanks quite literally onto their lawn.

Or so the plan goes.

However, I don’t think the Prince will be building this house as a quaint curio, a throwback to a bygone age. It will be conceived as an alternative, competing vision of what an eco house should consist of. And in doing so, a gauntlet is being thrown down. For there is a small but growing band of sceptics who think that the Code for Sustainable Homes is a deeply flawed manifesto and that its version of zero carbon is too narrowly defined and in some instances just plain wrong. It’s energy efficiency measures are so exacting that they have never been achieved anywhere in the world on a mass scale, ditto its airtightness requirements. And yet, on the other hand, it has almost nothing to say about the embodied energy of the products used to build zero carbon homes. And other aspects of the Code are so restrictive that many perfectly serviceable building sites will no longer be developable. It is one huge experiment that has yet to be tested in the real world to see if it’s achievable, or to see if the hoped-for energy savings actually materialise. So there is plenty of scope to rewrite the Code before it becomes set in stone. Or should that read polyurethane.

There is a lot at stake. At the moment, the Roundheads at the BRE have set the agenda: they wrote the Code for Sustainable Homes, after all. The Cavaliers here are the Princes Foundation, seeking a softer, more organic homebuilding charter. The BRE Innovation Park looks set to become the battlefield. One can only hope it all doesn’t dissolve into a pointless mud slinging exercise, and that some compromise pathway may emerge. But I wouldn’t count on it. For the truth is that the Green Building Movement — indeed the whole building industry — has long been split down the middle by these two conflicting approaches, and the fact that they will be presented here, side by side, on public display may simply serve to highlight the schism. In any event, I am already looking forward to Offsite 09, even if one bit of it will very definitely not be.