So how much does it cost to build a house? That's probably the number one question anybody who professes to be an expert about self-build gets asked.

My number one response is to beware of answers that look too good to be true - they usually are. The simple fact is that in order to give an accurate response to the question, you need to have hugely accurate details about what the house will be. How big? What materials? Who will build it, and where?

The more detail you provide, the more accurate the answer you're going to get back. We have battled with this problem for many years both in the magazine and online. And, of course, it is perhaps the most critical part of the whole process. Without an accurate idea of how much the whole thing is going to cost, you've got next to no chance of working out if you've got enough money to finish the thing at all. Grand Designs-style disaster ensues.

So what do you do? For several years the magazine has published a rather complex-looking grid of cost/m2 build cost guides. These are based on the RICS Rebuilding Cost Guides that insurers use for their purposes, adjusted to take into account our self-build variables and are, I believe, pretty accurate as a first port of call. You work out what build route you're going to take, whereabouts your project is, the spec, and you can quickly find a cost per m² of floor area. I remember using it back when I self-built and it was something like £5-10/m² out - which over a £900/m² project is pretty impressive. You can find the latest guide online here.

Obviously within a 'broad range' estimating tool like that, it's difficult to take account of individual factors ranging from design choices to leftfield materials. That's where you need to invest a bit of time, effort (and money) in getting a more accurate and detailed breakdown of costs. One such service is the My Building Project Estimating Service (full disclosure here: a partnership between Homebuilding's parent company and estimating software company HBXL). For a few hundred quid they will take your plans and give you a full, 30-40 page breakdown of every materials quantity going into your project, along with required labour, and provide a very accurate cost estimate. People who have used the service in the past claim it's accurate to within 3% (not bad when a typical contingency fund is 10-15%).

It's kind of a way to get the benefits of a Quantity Surveyor without paying the £1,000s usually required to employ one for yourself. Swallowing a hit of £100s at the early stage is difficult to take, but the results of an accurate, incisive cost estimate might just be the difference between your project succeeding and a Grand Designs outcome.