Long legged Sac Spider (Cheiracanthium mildei)
Alternatively this could be the native Agrarian Sac Spider (Cheiracanthium inclusum) aka the Black-footed yellow sac spider. The two species can really only be identified when pinned out, dead and under a microscope but geographic range and the fact that was inside a building suggests C. mildei is most likely. The long-legged sac spider is another immigrant from Europe and Africa but has become naturalised.
They they are active predators, feeding on a variety of arthropods such as insects and other spiders. Prey detection may involve detection of mechanical vibrations of the substrate, and vision seems to play an insignificant role. During the day, they retreat to small silk nests similar to those used for reproduction. A new nest, which may be completely closed, open on one side, or open on both sides, is built every day in under 10 minutes