As we hear from various experts about building to the Passivhaus standard, it's worth considering how one might go about adapting an existing building to the EnerPHit standard. It's a much bigger task as you will be working with what you've already got and, perhaps, be trying to maintain appearance or certain features. Timing when you make your home more energy efficient is important, too.

If you're buying a house with a retrofit in mind, at least you can choose a property that lends itself to the project. Firstly, you need to be sure that the house is worth saving! You will put yourself under a lot of mental and financial stress if you only discover this midway through the build. So when looking at a potential property, think how you might retrofit it from the outset. It's going to have to be one of the main criteria on your brief, along with price, location etc.

• Get someone you trust, a builder or surveyor, to give you professional advice on how well built the house is. If a builder buys a house, you can guarantee they will look at the property with very different eyes to the average punter.

If you want to get your house certified to the EnerPHit standard, look at site shading.

• Is the south elevation blocked from the sun? If so, this is not going to help with solar gain.

Warning bells might also ring if you find a property with a concrete floor and low ceilings. You probably won't want to rip out the concrete slabs underneath your house in order to insulate the floor, so you'd be insulating on top of it. In this case, low ceilings would make it difficult or limit the increase in height. If you raise the height, you'll probably have to remodel the doorways, which will be a big expense. If it's two storeys or more, you may even have to adapt the stairs.

• If you can find a house where you can insulate the floor without significantly altering the finished floor level, that will save you money.

Ideally you want to insulate externally, so some exterior finishes lend themselves well to this whilst others do not. Buying a house that's got a white rendered finish is probably easier to replace with insulation and a new rendered finish than if you purchased a beautiful stone cottage that you won't want to cover up.

If you're buying a property in a conservation area, then you're unlikely to be externally insulating. Thus you will be internally insulating the walls, which can be far riskier from the point of view of moisture building up in that wall. At this point you must choose your insulation products a lot more carefully.