SITTING IN her London kitchen with a mug of tea, Katie "KT" Tunstall is, in every sense, sparkling. Having toured the world practically non-stop for the last three-and-a-half years, the singer-songwriter from St Andrews is about to see more of her dreams come true: this month she officially becomes an arena-sized artist at home, playing her biggest-ever Scottish concerts, at Glasgow's SECC and Aberdeen's AECC.

Size, of course, is no indicator of talent. But it is an indication of appreciation, and 32-year-old Tunstall - from her days living on the dole with folkies in Fife cottages, to busking in Edinburgh - has worked long and hard to have people listen to her music. Winning the 2006 Brit Award for Best British Female Solo Artist is all well and good, but Tunstall is - in a bums-on-seats, hands-in-the-air, singalonga-KT kind of way - a people person.

Her second album, Drastic Fantastic, is selling well all over the planet. It will have to go some way to catch up with the five million sales of her debut, 2004's Mercury-nominated Eye To The Telescope, but, to be fair, it has only been in the shops since September. When you promote your records as hard as the perennially-gigging Tunstall - and when it took you until almost the age of 30 to become a success - you're always thinking about the long haul.

"And look," says Tunstall, waggling a finger at me and offering more sparkle.

It's an engagement ring: her boyfriend Luke Bullen, drummer and musical director in her band, popped the question during a surprise visit to her family home in Fife on Christmas Day. They'll be getting married - international touring commitments permitting - in Scotland later this year. They've been living together in this flat, originally bought by Bullen, since 2004, 18 months into their relationship. "I'd finally qualified for Luke to invite me to move in," she laughs.

Tunstall says she has been enjoying performing the new songs from Drastic Fantastic. Hardly surprising considering she promoted the slow-burning Eye To The Telescope for two years - in 2006 she performed in America every month of the year.

"To me it feels sexier than the first album," she says. That notion is underpinned by the glamorous shiny frock she sports on the cover, although she adds: "The word that I've come to associate with Drastic Fantastic is adolescent' - it's more adolescent than the first one. I don't know if Eye To The Telescope was younger or older. Maybe it was both, mature and innocent at same time. But this one is a little bit more carefree. You know, there's a song about teenage sex, Little Favours. Obviously legal age!" she adds with a hasty grin. "But the subject matter of the lyrics has taken me in a lot more directions."

Some fans on Tunstall's message boards took issue with the second album's glam look. Why did she go down that route?

"It wasn't intentional, to be honest. It happened very naturally. I'd been feeling pretty uncomfortable about the image side of what I do. I really didn't feel I'd found a place where I knew what I wanted to look like." After she had the idea of her playing a "mirrorball guitar" on the Drastic Fantastic sleeve, she found that this fitted thematically with a spangly dress she had picked up on her travels.

"You know, I'm really into the theatre," says Tunstall, who performed as a child at St Andrews' Byre Theatre and who later studied theatre at college in England. "That's where I came from initially, and I really love a good show."

Her glam image is not, then, about distraction, but rather a question of enhancement and projection? "Definitely. Well, it's about a respect for the theatrical nature of putting on a gig. I'd much prefer to put on a mirrorball frock rather than have video screens or moving backdrops and all of that - that's distraction to me. I mean, a band like Radiohead, they've got fantastic visuals. But their music is experimental, and that does enhance the music you're listening to. However, with my stuff it's very much about the lyrics and the personal connection to what's being said. And I don't want to be showing a video while I'm trying to communicate with people."

Tunstall has, it seems, known her own mind from an early age, and was always encouraged to be an independent thinker by her academic parents. They adopted her when she was a baby; Tunstall has since traced her birth mother, a part-Chinese woman who still lives in Edinburgh, but has never traced her Irish birth father. ("I won't be devastated if I don't find him because there is nothing missing," she has said.) Growing up, she and her two brothers excitedly accompanied their physicist father to the observatory at St Andrews University - the title of her debut album draws on memories of childhood stargazing.

There wasn't much recorded music or TV in the Tunstall household: one of her brothers is deaf, so any added noise made it even more difficult for him to understand what was going on. Instead Tunstall immersed herself in theatre, and hiking and walking, and learning to play instruments (piano, then guitar). She also became interested in travelling: at 16, with her parents' blessing, she went off inter-railing round Europe. "My parents have always been intelligently liberal with me and my brothers," she told me in 2005. "They knew it wasn't good for me to be hanging about on the streets when I was 16, so they let me go to the pub. I didn't come home a mess, I'd just go out and enjoy myself."

Aged 17, rather than spend a pre-university gap year bumming around, she spent a year on a scholarship to a school in Connecticut. "I was the all-singing, all-dancing Scottish quota they had to fill. Everyone else was into politics and law and went to nice schools and wanted to be prime minister. I came from Scotland and I wanted to be in plays and write songs and stuff."

This enthusiasm kept Tunstall's dreams alive through the myriad musical adventures and misadventures that followed: the record company interest when she was 19 (the A&R guy in question stuck by her and is now her manager). The period hanging out - skint - in Fife as part of the Fence Collective of musicians. The pub gigs she put on in Edinburgh in 2001 under the billing Acoustic Extravaganza. The £12,000 publishing deal that took her to London in 2002. The trip to New York at the end of that year at the request of legendary record company boss Tommy Mottola, who wanted to sign her to Columbia - and the shattering of that dream when Mottola left the company, and all bets and deals were off. And, finally, signing her current record deal in September 2003 - but even then, waiting a year to release Eye To The Telescope. Then another year for it to become a bona fide success.

All of which, combined with her solidly grounded upbringing, has helped keep Tunstall a seriously sorted woman. I met her at the London flat last summer too: she told me then that, for all her burgeoning fame, she was determined to stay "normal". She still talks regularly to her oldest friend from St Andrews, whom she met at a puppet show when they were three. "She's a clinical psychologist now - a good person to call when I'm having shit on the road!" And she still has lots of friends in Edinburgh. Indeed, she recently bought a flat in Edinburgh, near the top of Leith Walk.

"I just feel like a wanker, going out and trying to make friends with other famous people. Unless I meet someone through someone. Like Mika for example - we met having lunch somewhere and we had a really nice chat and we keep in touch. I don't know him well, but it was a natural way of meeting someone else who does what I do."

Aside from her engagement, there is another reason for Tunstall's twinkly good humour this morning. The builders have finally finished work on this flat in Harlesden, northwest London. It's been a long old haul renovating the fairly modest first-floor pad. She and Bullen had a false start in October 2006, after their architect supplied them with wonky drawings. Result: the builders knocked a hole through to the neighbours' flat while following his plans for a new staircase. It took them until March last year to "get rid of him".

Tunstall didn't want a normal renovation; she wanted a top-to-bottom eco-makeover. She strives to be environmentally conscious, an attitude her thrifty and outdoorsy parents instilled in her and her brothers from an early age. In America, her tour buses run on biofuel, and she sticks a levy on her ticket prices.

"I'm offsetting people's journeys to my gigs now - it's an extra 50p-to-£1 on a ticket. The venue's carbon output is very little. But I'm actually neutralising your journey to the gig, by offsetting - tree-planting, investing in renewable energy Having a nomadic existence can create an enormous amount of waste. So on the tour bus we use recycled plates and cups and are not using Styrofoam ever," she insists firmly. Her merchandise is also eco-friendly, the T-shirts made entirely from organic cotton.

She is even cutting down on her "rider", the normally lavish array of food and drink that an artist of her stature is provided with in their dressing room at shows. "Some pitta bread, some hummus, some fruit and some booze, that's it. We have dinner before the gig and there'd be this outrageous amount of food that just went in the bin. Really, really bad news."

Tunstall is applying these values to her home too. When I met her there last summer, there was a gleam in her eye as she imagined the Gas Saver gadget that would re-use heat wasted up her boiler's flue, as she talked excitedly of toilets that would cut her daily water usage by 20% and taps that would save 75% of water used in both bathroom and kitchen sinks. She was also looking forward to erasing the last remnants of the previous tenant, an unemployed bachelor.

"There was absolutely no female touch whatsoever," she said with a wince. "Nasty carpets. Really bad, fake parquet flooring in the living room. Terrible bathroom suite - the top of the cistern was broken in half. Very, very old kitchen with fake wood in a beautiful lichen-green colour." She grimaced. "It was actually more aquamarine."

With her personal belongings packed off to the temporary flat borrowed from a musician friend, Tunstall made herself busy performing in America while the builders moved in, took the roof off and removed most of the walls. By the time we catch up again, she has toured round the world twice. And the builders have worked wonders on the flat.

In the living room now is a three-piece suite recycled from her parents - it was made by her great-grandfather. "When I was a kid, it was always covered in this stuff called uncut maquette', which is the most horrendous itchy bastard fabric!" laughs the woman who calls her guitar effects pedal Wee Bastard. "And because it was so nasty, Mum and Dad always covered the whole thing. So we got it re-upholstered in a place round the corner."

In the former loft space, Tunstall shows me her favourite innovations: a huge picture window and 11 solar panels - buying and fitting the latter comprised a significant part of the £170,000 budget for the whole renovation. This is now her music room. It features a 1901 Bechstein piano she and Bullen bought on eBay from a bloke in Norwich. The hairiest moment of the whole renovation came when a crane parked in the street hoisted the instrument over the roof, over the solar panels and in through the picture window.

It's been well worth the investment, she reckons. When she and Bullen finally come off the road, they have a home to return to that they know fits their needs, and - as they see it - their obligations to live environmentally soundly. Up here in the loft, she and Bullen can play (quietly), and they can record. "We could make a green' follow-up to Acoustic Extravaganza," she says eagerly, referring to the album she recorded in a friend's living room on Skye over Hogmanay 2005-06 (and named in honour of those long-ago Edinburgh music nights she hosted). In fact, this enthusiastic imbiber of the water of life already has the perfect instrument.

"This is my Talisker guitar," she beams, cradling an acoustic. "It's made from old whisky barrels. It's my eco-friendly guitar."

KT Tunstall plays the SECC, Glasgow on Saturday and AECC, Aberdeen on Sunday