To find and restore one Christopher Wren church tower as a wonderfully eccentric family home is one thing. Having done so, to sell up and start work on another smacks of obsession. So I go to see Kate Renwick, a cheerful Irish-American lady, in her eyrie in Christchurch Tower, near St Paul’s Cathedral.

It is a fantastically improbable place to live, a Gormenghast for the Wallpaper generation. She opens the tall oak doors; I step inside and stop dead in something approaching wonderment. “You like a project,” I observe. “Don’t I just,” she replies.

Renwick is an IT consultant, a widow with two grown-up sons — one working in the City, the other a student — who suddenly found herself doing something extraordinary. “I certainly didn’t go looking for this,” she says. “I just happened to see it in an estate agent’s window. My elder son said, ‘You have to do it.’ It was a challenge. And once you’re into something like this, you’re in.”

We are sitting in the window of her kitchen/breakfast room on the second of 11 levels. Her sons’ bedrooms and studies are on levels five and six, while hers takes up the whole of eight. You find reception rooms on three and nine. There’s a library on 10, with bathrooms and toilets on four and seven. On floor 11, you can walk out into the open air and make a circuit round Wren’s reassuringly high and hefty stone balustrade.

Once up there, you are surrounded by office buildings. Just to your south are Paternoster Square and the dome of St Paul’s. To the west is the spindlier dome of the Old Bailey. Smithfield market, with its restaurants, is nearby, as is the way to the Millennium Bridge, across to Tate Modern. Busy roads pass close by, but, once inside, you are not aware of any of that.

Being in a church tower, none of these rooms can be large, but add them all up and you have a vertically arranged house of 2,288 sq ft, drenched with daylight on all four sides through a variety of windows, including the louvred former belfry. On floor three, there’s a tiny lift — just big enough for two — that runs to the upper levels, as far as 11. If you’re feeling brave, you can carry on up the steadily narrowing tower on fold-out aluminium ladders, though you’d need to be a steeplejack to get to the pinnacle. This home — for once, unique would be a proper word to describe it — is for sale for £4m.

Wren designed and built the church in stages between 1667 (the year after the Great Fire of London) and 1687, though they got round to finishing the tower only in 1704. There it remained until Hitler’s bombs destroyed the main body of the church during the blitz. The elaborate Portland stone tower was damaged, but it was restored after the war.

The architectural historian Sir Nikolaus Pevsner proclaimed it “one of the most splendid in London”, and he was not wrong. The site of the main church building is now a well-kept public garden laid out on Wren’s plan.

Renwick shows me around. You would think it would be simple — here’s a floor, there’s another floor — but it is surprisingly complex, and you can get lost. The lift has doors on both sides, and some of the levels interlock. Nothing corresponds to any of the few floors the tower once had.

So, although you can scramble into the original circular stone staircase at various points, you find a proper door only in some places. In others, there is a little hatch you can pop through, like a rabbit down a hole. Nicholas Boyarsky, the architect, designed it as a place where every level feels different, which makes sense, as the tower changes shape quite a bit as it rises. The only common factor is daylight, which is why there’s a fair bit of Perspex to be found — in spiral stairs, for instance — along with vertiginous glass floors, and some fine built-in oak bookshelves and suchlike.

The tower was in a sad state when Renwick began the project four years ago. Only the lower levels were in use, as badly converted offices for a charity. So she gutted it and started from scratch. “Banks don’t like to lend money on unusual old buildings,” she says. “They didn’t have any comparable to this. But now, when you’ve invested so much time and effort, it would be a waste not to reuse the knowledge you’ve gained.”

By which she means she is about to do it all over again. She has found another Wren tower — St Mary Somerset, a few streets to the south — on the Buildings at Risk register. So we stroll down there, undo the padlocks and step into a dusty, elegantly domed vestibule left empty for so long, even the ghosts have upped and left.

The church was demolished in 1869, but the tower saved, as it is by Wren and has some eccentric pinnacles on top. Thus it stood, unloved, for all those years, until Renwick decided this was to be her new home. Once again, Boyarsky is her architect. “You can see what you have to do with it,” she says of this simpler, less ornate building. “The gardens aren’t as big, but from up on top you’ve got views of the river. And it’s more original.”

The money from the sale of the first church tower will be ploughed into the second one. During the year the building works take, she and her family will live in a flat in the Barbican that they kept on from before their days of ecclesiastical conversion.

I was still slightly puzzled as to why Renwick, who is not a developer or a conservationist, wants to tackle such historical projects. “In my work, computers, everything is gone almost at once,” she explains. “But with buildings like this, you’re working with things that will last for hundreds of years.” She’s right. In an age of digital ephemera, there’s nothing quite so satisfyingly permanent, so unconventionally analogue, as a Wren tower.

- Christchurch Tower is for sale through Savills, 020 7730 0822 or 020 7226 1313, www.savills.co.uk

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