The move of the Royal Observatory to the site of Herstmonceux Castle, Sussex, began in 1948 and was completed by 1957. The Observatory occupied the Castle building and hutments, and a large building programme was led by architect Brian O'Rorke to complete the necessary new structures. The Herstmonceux site was home to the RGO until 1990 when it moved to Cambridge.

This series includes site plans, plans of Herstmonceux Castle, and plans and drawings of the new RGO structures on the Herstmonceux estate, including the Equatorial Group of Telescopes, Meridian Group of Telescopes, West Building (housing the Time and Nautical Almanac Building, Chronometer Workshop, Electronics and Engineering Workshops etc.), and Isaac Newton Telescope Dome. It also includes plans of the temporary hutments where staff were housed until suitable accommodation became available.

Notable are the colour drawings of the staircase of Herstmonceux Castle (RGO 101/3/2/1) and colour elevation and section drawings and plans of the Equatorial Group of Telescopes by Brian O'Rorke (RGO 101/3/5/1/18-22).

In addition to building plans, the series includes technical drawings of telescopes and instruments, including the Photographic Zenith Tube and Yapp 36'' Reflector. Many of the plans and drawings in this series were created by external agencies, including Brian O'Rorke and Grubb Parsons and Co.