1-3 Willow Road, Hampstead (1940) by Ernő Goldfinger.

Number 2 (the middle house) was Ernő Goldfinger's own house for many years. It's now owned by the National Trust, and they do tours for visitors on Saturday afternoons (and possibly other times). It's well worth a visit - the interior is done with great concern for materials and attention to detail. It's very successful, and much less austere and difficult than you might imagine. Likewise, you really need to see the outside of the house at close quarters to fully appreciate it - I particularly liked the wood and translucent glass front door and hall area.