Primrose Hill is a northwest London neighborhood famous for its topography — the hill offers skyline views — and envy-producing, village-like charm. The district has Georgian- and Victorian-era terrace houses and many leafy trees. But even locals may not know of a little street that wends through it like a country lane.

Klaartje Quirijns, 56, a filmmaker, and Hannes Witteveen, 55, a lawyer, once counted themselves among the ignorant. For a decade, the pair, who are Dutch, lived with their two daughters around the corner from that lane without having a clue that it was there or that their future home sat on it.

But as Ms. Quirijns once said in an interview, “All my films arise from the same question: ‘What are the deceptive worlds we don’t get to see?’”

The house they would buy in 2020 — after a lifetime of renting, they hungered to be homeowners — was an odd duck among the preening architecture. Square, flat-topped and brick, it looked like a baby bunker.