(Covers information from the S.D. Perry novels)
The Spencer estate was a mansion built in the Arklay Mountains.
The mansion's construction was planned and overseen by George Trevor, a noted architect from New York, as a retreat for the Umbrella CEO, Spencer. George disappeared shortly after the mansion's completion, promoting an urban legend that he went mad during its construction and got lost in the corridors before finally dying from malnutrition. This was used as an explanation for why Spencer never moved in.
Officially, Spencer intended the mansion to be a guest house for Umbrella executives, but abandoned that idea soon after completion, moving the company's headquarters to Europe. Umbrella used the mansion from time to time to store chemicals, advising people to stay away from the property as a safety hazard. In reality, it was using the mansion as a front for its secret biological experiments in an underground laboratory complex.
When the t-Virus escaped from the mansion, S.T.A.R.S. found the mansion to be at the heart of the region the cannibalistic deaths and animal attacks were taking place. With elements of S.T.A.R.S. believing that a cult was using the mansion as a hideout, Umbrella tried to limit interference by alerting to S.T.A.R.S. that they had someone investigate the house recently and found nothing. When S.T.A.R.S. succeeded in destroying the mansion, taking with it many of the T-Virus - infected organisms, Umbrella explained away the explosion (a bomb) as being a spontaneous ignition of the chemicals in storage.