Tim Rushby-Smith walked out of his home to go to work on April 1 2005. He finally got back again 18 months later. And when he did return to his flat, in east London, he was in a wheelchair. Tim, a tree surgeon, who ran a garden design business, had fallen out of a tree and broken his back.

"I saw this shape in my peripheral vision, falling," recalls Penny, his wife and business partner, who was working with him that day. "I realised that was Tim falling on to a roof. At that point, I thought, 'Oh bloody hell, I bet he's got a broken leg. How inconvenient.'" But then, she got a better view. "I could just see he was lying there, really still."

Tim, 39, a big outdoorsy guy who played football, and liked climbing, the type who went mountain biking and to the pub with mates, and who, most importantly, was just about to become a father (Penny was five months pregnant) was now a "T12 complete": paralysed from the waist down.

For the first few weeks, he was flat on his back, completely unable to move. He suffered the worst kind of claustrophobia at being trapped inside his own body. Any attempt to move his legs, or even to move in bed, led to a lurching panic attack.

He thought of all the things he'd never be able to do again. Hearing Dobie Gray's Out on the Floor, on the radio, took him back to his former self dancing in the 100 Club. He thought of how he had lost his future as a father, a husband, a provider. He couldn't imagine how he'd be able to do anything at all. "Everything in my life had turned upside down and disappeared," he remembers.

But those who know Tim say that when he sets his mind on something he generally achieves it, and now, three years later, he plays hide and seek with his daughter, Rosalie, two, drives a Renault Kangoo, has been on holiday in Australia (Penny is from Sydney), cooks, plays wheelchair tennis, and has seen Maximo Park at the Brixton Academy.

Moreover, he's written a book, Looking Up, an account of learning to live again with sudden disability. Based on emails to friends, and a diary he kept during his four-month stay in the National Spinal Injuries Centre at Stoke Mandeville hospital, it charts Tim's rehabilitation: the drugs, operations and tears. This is heavy material, but also, at times, unexpectedly funny. It is also remarkably frank. We learn, for example, that worse by far than the pain of a broken back, is the agony of "compacted constipation" (a side effect from the shock of the accident and the copious amounts of morphine).

The detail in Looking Up is quite extraordinary. But then Tim Rushby-Smith is the sort who likes to know things. "Tim wanted to know everything from the very first day," reveals Jackie Bailey, a peer support officer at Stoke Mandeville. "Some patients take a lot of time to take on board any information I can give them. But Tim was full of questions."

I meet Tim and Penny at their flat in Hackney, east London. Tim sits tall in his wheelchair. This is because he is 6ft 2in, and also because his "wheelchair role model" is Justin, a young man he met in hospital, who looked straight and slim and comfortable. Not like others who "almost melted, as the chair sags and they sag with it". It's part of body image, he explains, "adjusting my self-perception and also trying to get back to my six foot two-ness. I don't like looking up people's noses all day."

He is wearing jeans and a T-shirt, but says he's yet to find the perfect pair of trousers. "As well as being roomy enough to sit down in them all day, I need them to make my legs look big and normal, rather than all flat, like someone's pulled the plug out the bottom, and they've all deflated."

Tim Rushby-Smith, you quickly realise, is an ordinary person, who's been through something extraordinary, and is now learning to be ordinary again.

"A lot of the books I read on the subject were about people who'd suffered some terrible injury or setback and then gone on to climb a mountain. You know, all these really exceptional stories, which are incredibly inspirational, on one level, but for me, it didn't have that much relevance."

Which is why he wanted to write Looking Up. "I wanted to create the notion that it's possible to have a normal life again," he says.

What you get from Looking Up are Tim's struggles to open a window, take a shower, buy ice cream. There's also plenty of sex in the book. It starts on page 45, when he is still in hospital. "Now that I'm paralysed from the waist down, does handling my ... well, myself, does it feel like I am handling some else's ... self? Answer: I'm not sure, having not tried it." He goes on to explain that it feels strange - a bit like holding your own arm, when it's gone to sleep. He also worries that no longer being able to ejaculate has caused him to develop acne on his forehead (messed-up testosterone levels).

Much later, at home, there's a traumatic encounter with ErecAid, a vacuum pump that creates an erection, and Viagra, which turns out to be so effective you almost cry: "Sharing that intimate pleasure with Penny, having those moments lost to passion, connects us to our old life together, makes the before and the after into a whole, single experience."

Did you ever worry that it was too personal? "You should see the stuff that didn't make it into the book!" he laughs, explaining that he ran everything past Penny first. "Let's face it," she says, "you have to write about sex and you have to write about bodily functions, because these are the major things that change in this situation."

They both agree that Penny, 34, is the stronger of the two, and has been crucial in keeping him sane. She seems to have that zestful talent for creating a good life even in adversity. When her husband was in Stoke Mandeville she would arrange intimate dinners for two, with home-cooked meals, using fresh ingredients.

The first thing Tim did when he saw Penny, after the accident, was to apologise. It's a common response, apparently. "It's that sense that you've let those close to you down by letting something happen to you," Tim explains. "That you're exposing people close to you to this awful experience."

They met through mutual friends in 1997, when Penny was on a working holiday in London. She says they are perfectly matched. They certainly share creative backgrounds; his in Islington, hers in Sydney. His father, now retired, was a studio manager for Radio 3 and a composer; his mother, Uli Rushby-Smith, is a literary agent (his literary agent, in fact). Penny's father is a furniture designer, and her mother a guide at the Royal Botanic Gardens, Sydney. Both Penny and Tim studied fine art, though he left after a year - "I didn't appreciate what I had there" - to work in odd jobs including being a printer and telephone engineer. But always painting.

"We have a lot in common, though he has a better sense of humour," says Penny. Tim, for his part, was impressed by her gumption. "When I first met Penny she'd just spent three months cycling around Europe on her own. I'd barely got out of Islington."

They married in 1998, and set up their gardening business in 2002. Tree surgery, he explains, was a recent addition. "I've always enjoyed climbing," he says. "I find height focuses the mind." Plus, with Penny being pregnant, it made sense for Tim to have something he could do independently. So in the autumn of 2004, he applied for a four-month course at Capel Manor College, Middlesex. He left feeling galvanised.

With the future looking bright, they took a five-week holiday to Australia. They came back to a cold spring, and their first post-holiday job: to build a tree house, for a client, in Muswell Hill, north London.

Tim does remember falling the six metres, but didn't feel any pain. "It wasn't a feeling of pain, it was a feeling of force, a lot of force." Penny told him to concentrate on listening to the birds, while they waited for the air ambulance to arrive.

After the accident, Penny put the gardening business on hold and took early maternity leave. She visited Tim every day in hospital, eventually moving into a flat in Aylesbury near the hospital, so they could be together when their baby was born.

"Being pregnant gave me a real focus," says Penny. "When I was feeling alone and upset, I had the baby to talk to. I felt comforted by its presence," she explains.

"It gives you a future," echoes Tim, "a positive life event in front of you, and that was fantastically important."

The first days in hospital were the darkest. "In the morning, you'd wake up and open your eyes and remember where you were." Tim pauses. "I had some dreams where I was walking and I'd say, 'Tell me I'm not dreaming.' And they'd say, 'No, you're not dreaming. It's all real.' But it wasn't."

The turning point was meeting Jackie Bailey, a few days after arriving at Stoke Mandeville - the first person he saw in a wheelchair. When she was 17, Jackie fell off a climbing wall in a gym and missed the crash mat. Now 45, and paralysed from the waist down, she is living proof that life goes on, having had two children, plus numerous holidays, including skiing and horse riding. "Patients go through a grieving process," she explains. "They think they've lost the person they were, and actually part of rehabilitation is to learn that they are still the same person and can still do a lot of the same things, but in a different way."

She says the biggest challenge for patients isn't necessarily rehabilitation, but finding somewhere to live afterwards. "There is such a shortage of accessible housing, so once we've built people up to go home, they have no home to go to. It's very frustrating."

Here, Tim was lucky. He'd bought his flat in 1994. "It was the only thing that enabled us to be financially secure," says Penny. "We could have been homeless if we'd had the kind of mortgages some of our friends have." They rented out their Hackney flat, which covered the mortgage, and then a network of friends clubbed together to pay the rent for their flat in Aylesbury. "They were extraordinary." The couple got a disabled facilities grant towards reconfiguring their flat, and finally moved back in October 2006.

Penny now works as a garden designer two days a week and is at college one day a week, when Rosalie is looked after by a childminder. As well as writing, Tim paints and he's a doting father. "Rosalie's an incredibly grounding influence," he says. "Whatever else is going on, she wakes up in the morning, and pokes me in the face. It just drives you on." Penny says another child is "medically possible, I'd hope, with IVF" but she wants to wait until Rosalie is more independent.

Has Tim changed? "He's always been very creative, but perhaps not quite as focused, and this has made him very determined." She says he is still in a lot of pain. "Sometimes it can go on for hours. I don't know how he does it. Actually, that's a pointless thing to say, he doesn't know how he does it, but he just has to do it, so he does."

She says she doesn't feel bitter or angry. Just sad. "I see all these couples walking hand in hand, or dads pushing pushchairs, and I feel sad for all of us."

"There's stuff that saddens me every day," echoes Tim - his chainsaw boots, for example, his books on pests and diseases in trees, the fact that he can no longer easily get into his back garden. "But I've learned not to think about it. It's just an elaborate form of denial, I suppose."

· Looking Up: A Humorous and Unflinching Account of Learning to Live Again with Sudden Disability by Tim Rushby-Smith is published by Virgin Books at £7.99. To order a copy for £7.99 with free UK p&p go to theguardian.com/bookshop or call 0870 836 0875.