From the debate so far, you'd think that consequential improvements were designed purely and simply to get up the noses of aspirational, squeezed-middle, Daily Mail reading home improvers. They weren't. There is a good case for them and it's not being heard.
At the heart of the matter is the question of energy saving and the law of diminishing returns. It states that the first inch of so of insulation saves a mass of energy, but each subsequent inch saves less than the one before. This is expressed in mathematical terms in U value calculations which show that if you want to halve the heat loss, you have to double the amount of insulation.
It's relatively easy to halve the heat loss of an existing wall with no insulation in it. Adding around 50mm of insulation will reduce the U value from around 1.2 to around 0.6 (that's half). It's much more demanding to halve the heat loss in an extension which is being built to modern standards: getting a wall from a U value of, say, 0.3 down to 0.15 would involve widening the cavity from around 80mm to nearly 200mm, and forking out for 120mm extra insulation. There are other details to consider here as well, but for simplicities sake, I'll ignore them.
In doing this, your extension may have halved its energy loss but it's done it from a much lower starting point. The total energy saved in the new extension will be far less than you might save in the existing house where you've only had to add 50mm of insulation. You've also, incidentally, lost a significant floor area from your extension as the walls have to be so much wider to cope with all that insulation.
The financial calculations are even more compelling. Adding 120mm of extra insulation onto a 30m2 extension is complex and relatively expensive: it could easily add a couple of thousand onto build costs. Cavity wall insulation on an existing house usually costs between £300 and £500. The cavity wall insulation will result in far more energy being saved, and have a far greater effect on fuel bills. Like an order of magnitude.
Financially, it's a no brainer. It makes sense to insist that the existing house is upgraded rather than pouring money into building an ultra-low-energy extension.
There is more. Any energy saving you might hope to get in the newly-built extension will be mostly lost via the existing house, meaning that the expensively constructed low-energy extension will probably not save any appreciable amount of energy at all. And the whole house now risks being uncomfortable to live in, neither fish nor fowl. It would be like wearing a coat where the left side is made of a thin cotton and the right side of thick sheepskin. What a crazy design? Who would choose such a coat? Who would choose to live in such a house?
Like it or not, consequential improvements make sense, at least when the house is being extended. If it was to really have teeth, Part L would be insisting that anyone extending the heated floor area of an existing house should ensure that the overall heat loss of the new should be no more than the old. Now that would be draconian (but it would also make a lot of sense).
As it stands, the suggestions for improvements as set out in the Part L consultation are very watered down so that only four actions are required:
• cavity wall insulation (only if you've got an unfilled cavity)
• extra loft insulation (big deal)
• cylinder lagging (again, it's a nothing job, and over half the country doesn't even have a cylinder, they use a combi)
• draft proofing
It's very hard to see this lot costing £10,000 for anyone, unless you happen to own a huge country house. It's hard to see it costing much more than a grand on most homes. With or without the Green Deal, it's not going to be very expensive to install these and it makes sense it terms of overall comfort, if nothing else.
Up till now, Part L has tried to maintain some sort of parity of standards between new build and extensions, but it's reached the point when the two are about to separate because (as discussed) it makes little sense to build extensions to new build standards if the existing house is a drafty old pile. What should Part L do for extensions? Should it insist that they continue to ape new build standards (cost maybe £2,000 per extension, energy saving negligible) or should it branch out and insist that the equivalent money is spent in the existing house? The answer is obvious: consequential improvements are logical, sensible and justifiable.
200,000 extensions a year represents a huge floor area, comparable with the floor area arriving via newly built homes each year. Are we just going to give up on trying to get these homes fit for the 21st century? High energy bills are not going to go away anytime soon and Part L is one of the very few tools at the government's disposal to protect people from them. Ditch consequential improvements and you leave a gaping hole in energy policy.
As a footnote, I do think it was a mistake to add replacement windows and boilers as triggers for consequential improvements. I wonder whether they were placed in the consultation as a negotiating ploy, so that something could be conceded before the final version of Part L was published. If so, it has backfired badly. The principle of consequential improvements should be simple to grasp: if you chose to extend the heated envelope, you must improve the energy performance of the existing house. Adding window and boiler replacement as triggers muddied the simplicity of this golden rule.