Kevin McCloud has a lot to answer for. It is Kevin's fault that I have been on the verge of bankruptcy three times and have been suffering emotional strain for two and a half years.

Kevin is the tall, handsome, dark-voiced presenter of the television programme called Grand Designs, by which I became gripped when it first ran in the autumn of 1999. Grand Designs follows people through the vicissitudes and pleasures (mainly vicissitudes) of building their own homes.

And Kevin is such an athletically enthusiastic presenter, so sexily knowledgeable about architecture, that it is possible entirely to miss the point of the films.

This is that every single project takes far longer than expected, and everyone runs out of money. If I grasped this idea at all that first series, it was only to think that obviously these people weren't going the right way about it. So when an estate agent finally ran out of patience and suggested that instead of buying a house I might like to buy a patch of weeds and nettles down a semi-industrial lane in north London, I jumped at the chance.

If I couldn't find a home to fit my family, I would build a house around them, just like on Grand Designs. Perhaps Kevin would turn up to admire my taste in kitchen worktops and discuss modernism with me. Well, no, because although Grand Designs did approach my architects, they declined on the grounds that Kevin and his cameras would be too much of a distraction. So I have run out of money and watched the building work drift on without his help.

At least until today. Kevin recently completed filming for the third series of Grand Designs, and he has agreed to be interviewed on the site of my new house. Unfortunately there's not much to see: although the builders have been working since last November and initially suggested a handover date of 3 April, the building still doesn't have a roof. There are some internal walls, however, and I have high hopes of Kevin appreciating the radicalism and drama of the design.

Kevin turns out to be quite as muscularly articulate in the flesh as he does when climbing through rafters in his hard hat on television. I walk him through the ground floor - which does, admittedly, currently resemble one of those houses in Croatia or Turkey where chickens wander through rubble and bits of concrete rear up. He says he's 'afraid I can't comment on its "contribution to the built environment"' - and he says this ironically, the last bit definitely in inverted commas, which I find disturbing.

For reasons that are still not perfectly clear to me, my house is built of poured concrete. This means that the builders erect wall-shaped boxes, held in place by metal bars, and then pour concrete into the moulds. (They do this one at a time, though. It's not fast.) When the formwork is dismantled, the concrete is left. This is how the National Theatre was built, where the concrete has taken on the grain of the wooden shuttering (mine is smooth). But the method is rarely used for small buildings in this country because (a) it easily goes wrong, (b) it's laborious and can take about 10 days to build one wall, and (c) it reminds people of multi-storey car parks.

I begin to explain to Kevin the history of my relationship with this concrete - that initially I didn't like the idea of such brutalism, that I thought of it as oppressive, unyielding, forbidding. 'But presumably you're putting internal finishes on it?' he says, as if no one in their right mind would leave concrete looking like concrete. So then I don't feel I can finish my spiel about how I've come to find it sensual and tactile, because he obviously thinks the idea of living with raw concrete is deranged.

'It looks rather small,' I say, 'although it looked really big on the drawings.' Kevin explains that buildings seem to be different sizes at different stages of construction. 'It'll look bigger when it's painted.' To which I want to shout: 'But it's not being painted. It's a concrete house! The whole point of it is to look concrete-coloured!'

Clearly, I am not getting what I had hoped out of this. We turn to the interview, about which I have high hopes because there are questions I want answered. I want Kevin to tell me why I ever embarked on this risky, financially devastating project and why everybody I ever speak to about it still says, 'oh, how exciting!' I want to understand what it is about building a house that exerts such a visceral, emotional hold over the imagination?

'It's one of those adventures that people feel they can go on,' Kevin says. 'And if you're married and you've got kids and a nice job, it's more approachable than flogging the house and taking the kids out of school and sailing round the world.' In this country it feels definitely eccentric. But in Germany where there are many fewer owner-occupiers, 85 to 90 per cent of those that do buy have built their own houses. Even here it's less uncommon than you might think: the Grand Design offices were inundated with offers of participants for the third series.

The appeal of the programmes, as Kevin points out, is their human stories. 'I started out not at all interested in docusoap; what interests me is the architecture and design of the buildings. That's why I'm there. But I've come to realise that the buildings only exist because of human beings, and they reflect the characters of the people who built them. So even if you try to talk about just the building, you end up looking at the people who built it, trying to understand it from a human point of view.'

The human point of view, I say, is that the building overruns and there isn't enough money. 'Those are facts,' Kevin says. 'Overrunning is endemic to the building industry. And one of the habits of human nature that gets exposed in this process is that ambition, once let out of the bag, runs riot. People will delude themselves to the point of bankruptcy. The builders can have left the site and the mortgage company be on the point of repossessing and they still think they're going to live there.'

Mmmn, I say, thinking of various overdrafts. So... over-ambitious, in need of adventure without actually being very adventurous... and mildly delusive. 'You couldn't find an event or project or occasion in life which is so replete with optimism,' Kevin says. 'Self-builders invest a lot of emotion and ambition and hope in their projects. Somehow you displace a lot of your hopes for the future.'

I tell him that I feel inadequate compared to a lot of the Grand Designs people, who spend all their spare time manhandling breeze blocks and designing their own composting toilets. My main role in the current phase of my project is to come and look at it.

'You shouldn't underestimate the importance of looking at it,' Kevin says. 'Houses become an externalised dream, an externalised realisation of all your hopes and ambitions. And a lot of people do, I think quite justifiably, use the build as a way of changing their lives. Where you live now, maybe it's an older house, several storeys, total chaos - and here you'll have lots of storage, throw things out, change your life, be a modern girl, live in the twenty-first century - and that's a major change.'

He can tell! Just by looking, he can tell that I am not really a minimalist person. He knows that my modernist fantasy is fraudulently unsustainable.

Kevin McCloud wanted to go to art school, but was 'press-ganged' into going to Cambridge instead, where he did a degree in art history and discovered that he preferred writing about buildings to writing about painting or sculpture. His dissertation was on Bauhaus theatre design and, for six or seven years after university, he worked as a theatre designer.

He set up a design studio where, in the Eighties, he specialised in sculptural theatrical installations for people's houses. 'I was big on ceilings - a lot of stuff with fabric, gessoed, gilded.' He still has a design practice, in Somerset, which is where he lives with his wife and four children (in an ancient house on to which he has recently added an extension). These days he mainly designs lighting for other manufacturers, although he has also done furniture and tableware. He is the author of a couple of books on paint, and has one about palettes of colours (and their historical and cultural references) coming out next spring.

He started out on television with a couple of guest appearances on early editions of Home Front . In his early 40s, he seems to have arrived at a life that suits him very well: a couple of days a week filming, a couple of days at his design practice, a day in London for meetings. Cleverly, he has avoided getting caught up in the makeover craze.

He is chary of saying too much about his own tastes, refusing, for example, to single out any of the Grand Designs buildings. ('Everyone has their own favourites, and I've found it's easier to agree, otherwise it's a 50-minute conversation.') He will admit to being broadly keen on sustainability, and he is dismissive of little detached 'Lego' developers' houses. He dislikes 'overweening ambition, or buildings that pretend to be something that they're not. I like simplicity, directness and modesty in domestic buildings.' And he is slightly incredulous that anyone could want their build to appear on Grand Designs.

'People write in and say: "Our project's going to be on time, on budget, and have no problems." They tell us: "We can't believe how appallingly run those builds are that you film." It's incredibly foolish. They're revealed as just as inept as everyone else, just as incapable of holding on to it.'

'Why concrete?' he asks as we leave the site. 'Why not brick? Why not timber?' Oh well. He liked my insulation material. He found some of it piled up in what will one day be the sitting room. He said it just slots together and it's fantastic. At least that's something.