Claremont Square

Where it sits. Claremont Square is a small Georgian square in Islington, north London, set just off Pentonville Road between Angel and King's Cross. Terraced houses of dark stock brick line three sides of it, and the fourth side is Pentonville Road itself. What makes the square unusual is the middle: instead of a garden there is a large grassed embankment, fenced with iron railings and rising well above street level, which most passers-by take for a park they cannot get into.

The story behind it. That mound is a reservoir, and it is older than the houses around it. The New River Company, which had been bringing drinking water into London from Hertfordshire since 1613, built an open storage pond here in 1709, known as the Upper Pond, on high ground that let water run by gravity into the City. The terraces were built around it in the 1810s and 1820s, when Pentonville was one of London's first planned suburbs, which is why the houses all face inward at a body of water. The pond was covered over in the 1850s to keep the water clean, and the grassed roof you see is the result. It is still an operating reservoir in the London water network, which is why there is no public access and no gate. The houses are typical of their date and worth reading carefully: four storeys over basements, flat brick fronts, round-headed doorways with fanlights, cast-iron railings and coal-hole covers in the pavement.

On screen. Claremont Square is 12 Grimmauld Place. The exterior of the Black family house and headquarters of the Order of the Phoenix was filmed on the east side of the square, along the terrace numbered in the twenties, in Harry Potter and the Order of the Phoenix and again in Deathly Hallows: Part 1. In the film Harry is handed a slip of paper with the address, thinks of it, and the houses either side grind apart to let a black-fronted extra house swell into existence between them. That effect is entirely digital. There is no gap, no number 12, and nothing to find on the real terrace except a row of privately owned homes whose residents have been putting up with people photographing their front doors for two decades. The interiors — the hall with the mounted elf heads, Mrs Black's portrait, the drawing room and the kitchen — were built as sets at Leavesden Studios and share nothing but a doorway with the real street.

What you actually see today. A quiet residential square that takes ten minutes to walk around, with a reservoir bank in the middle and a view south-west across the rooftops from the Pentonville Road side. It is unshowy and, unlike almost every other Potter location in London, entirely undeveloped for visitors: no plaque, no shop, no marker, nothing. That is part of its appeal, and it also imposes an obligation. People live here. Photograph the terrace from the pavement opposite, keep your voice down, and do not photograph into windows or stand on doorsteps.

The wider water story. If the reservoir interests you, the New River Head site is a five-minute walk south-west, off Rosebery Avenue: the point where the New River ended, with the former Metropolitan Water Board buildings, the Oak Room, and the round pond outline still legible in the ground. The Islington Tunnel of the Regent's Canal runs under the neighbourhood a little further north, and the towpath at Duncan Terrace makes a good continuation. The New River Path itself can be picked up further north in Canonbury and followed for miles.

Getting there and timing. Angel Underground station on the Northern line is about eight minutes away on foot down Pentonville Road; King's Cross St Pancras is about twelve minutes east, which makes the square easy to attach to a King's Cross visit. The square is a public street, open at all times and free. Daylight is better than dusk here because the terrace faces the reservoir and the light on the brick is best in the late afternoon. It is a residential area with little in the way of cafés on the square itself, though Chapel Market and Upper Street, both a few minutes away, have plenty.

Worth knowing. The square has served other productions as a stand-in for anonymous Georgian London, and its combination of complete terraces and almost no traffic makes it a location scout's shortcut. That same quality is why it works on screen: it looks like the kind of address a very old, very secretive family would have kept for two hundred years.