Iam about to give you a guided tour of what is a remarkable property in many ways, but let’s not beat about the well-appointed, recently renovated bush here. You’ll probably have already noticed what is perhaps the most remarkable feature. It is the proximity of the words “onebedroom basement flat” to the figure “£895,000”. Whichever way you view it, this figure is what an estate agent might describe as “surprisingly large and roomy”.

The flat itself is part of a substantial house on one corner of Warwick Square, which is about five minutes’ walk from Victoria station. Technically, this is in the central London district of Pimlico, but it’s one of those places you would expect to have branched out and invented its own name, in the manner of Belgravia, its more upmarket neighbour. Was Warwick Garden Village already taken?

The square was laid out in the 1840s by a builder called Thomas Cubitt, who specialised in those huge white mansions that look as if they’ve been uprooted, brick by brick, from Brighton to cover the surrounding area. Back then, the houses of Warwick Square were mansions where men with muttonchop whiskers stood before large fires, holding their lapels and contemplating the vastness of their wealth. Gradually, as the supply of wealthy buyers, muttonchopped or otherwise, began to decline, some of the mansions were turned into flats. Number 23 seems to have been converted in the late 1920s.

The owner, Johanna Thornycroft, bought what the brochure insists on describing as the lower-ground-floor flat for £439,000 in 2003. It was dark, pokey, and seemed to have doors everywhere. In other words, absolutely perfect for Thornycroft, who had already decided that basement was all she could afford in this area. She arrived to view the flat with a pencil, a sketchpad and a renovations budget of £100,000. “I’ve never bought anywhere that didn’t have potential,” she says.

Thornycroft, you see, is a writer who specialises in interior design. She is the author of, among other things, Dream Homes, More Dream Homes, Dream Homes Country, Contemporary Home, Country Home, The Provençal House ... you get the idea. In a properly ordered world, she would be writing this, not me. Viewing her flat was like buying a second-hand car from Jeremy Clarkson, or ordering a takeaway from Michael Winner. Oh, and this is just a guess, but I’d say she is probably the only working interiors writer who was previously a tobacco dealer in Zimbabwe. I could be wrong, of course.

The core of the flat is essentially two rooms and an ensuite bedroom. Unlike many basement properties, it is reached through the main house, where the foyer looks like the communal area of a rather formal boarding school. It’s grey and gloomy, in an expensive sort of a way. For the real, £895,000 effect, you must head downstairs to the flat.

Thornycroft describes herself as a colonial. She was born in Malaysia, where her parents ran a rubber plantation. She has spent her life travelling around the world — New Zealand and various bits of Africa — and this is reflected in the decor. A collection of Shona stools is propped up in a corner. There is a mirror she had knocked up from a cornice stone she saw workmen dismantling in Cairo, where she also had a sofa and ottoman made. She has a collection of busts of famous men who are long forgotten: a French composer here, a poet there. Lenin, who had such a terrible effect on Moscow property prices, is on the mantelpiece, alongside what looks distinctly like Garibaldi.

To get some idea of the ripping out and rebuilding that Thornycroft undertook, nearly all of those doors are gone. The walls are a calm pastel shade, the floor is oak. As you come in through the front door, the wall on the right is covered in tongue-and-groove panelling (in a Fired Earth paint that’s best described as grey, browny, olivey). Because your attention is caught by the busts and the rest of the decoration, you probably won’t notice the door handle in the tongue and groove. That leads to the bedroom, which has a vaulted ceiling and leads in turn to a small dressing room and a bathroom. Another door from the bedroom takes you to a utility room and a separate store room in what obviously used to be the coal cellar.

The key, unique, £895,000 selling point, however, is the kitchen/dining area and the main living space, which are divided by large pillars. This gives the flat a feeling of space not habitually associated with the word “basement”. The kitchen, by the way, is Bulthaup, and is flanked by two dark wooden bookcases that used to be French wardrobes.

This will probably become somebody’s London pied-à-terre. Peter Young, managing director of John D Wood estate agency, claims there has been a lot of interest from French and Italian buyers. We British might turn our noses up at paying close to £1m for a basement, but, remember, location is everything, and the market in Warwick Square seems particularly unusual. I found two two-bedroom flats for sale there — one at £535,000 and the other at £1.15m. Set against that, £895,000 is barely above average.

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