Commentary on Sections Part 3: Planning Control. Section 62: Duration of outline planning permission
Outline planning permission establishes for the applicant whether a proposal is acceptable in principle before embarking on the preparation of detailed plans (“reserved matters”).  Unless provided otherwise reserved matters must be submitted for approval within 3 years of the grant of outline planning permission and development must be begun within 5 years of the grant of outline permission or 2 years from the final approval of reserved matters.