An archaeological watching brief was undertaken by Colchester Archaeological at Pinnacle House, 21 St. John’s Green, Colchester, during groundwork in advance of the construction of an extension on the north side of the property (within the walled precinct of St John’s Abbey). The groundwork included a series of narrow trenches for the foundations of the extension, a glazed link between the house and the extension, and a garden wall to the north of the extension. An existing wall was incorporated into the south wall of the extension. The foundation trenches were 500mm wide and, with the exception of the trench for the garden wall, were dug down to the natural subsoil. The trenches for the extension and the glazed link were between 1.3 m and 2.2 m deep, while the trench for the garden wall was approximately 800 mm deep. The trenches were dug by the contractor using a mini-digger with a toothless trenching bucket, and were monitored by a CAT archaeologist. On health and safety grounds, the recording of the archaeological deposits was done from the modern ground level looking down into the trenches. The natural subsoil (Layer or L3) was reached in the foundation trenches for the extension and the glazed link. In the latter it was fairly shallow, approximately 0.8-1.2m below the modern ground level. In the eastern part of the extension, it was 1.2m deep while, in the western part, it was approximately 2m deep. In most of the trenches, the natural subsoil (L3) was sealed by an extensive, homogenous, greyish-brown layer (L2). This was between 600mm and 1.5m thick, and was thickest over the north-western part of the extension. A small quantity of human bone fragments was recovered during the machine-digging of the foundation trench for the north wall of the extension. The fragments were well-preserved and included lower limb bones, vertebrae, and skull and mandible fragments, probably from one or more adult/adolescent individuals. However, the context of the human bone fragments remained unclear. They probably came from about 1.75m below the modern ground level, where further bone fragments were visible in the north section. It was uncertain whether the fragments came from an in situ burial or whether they were redeposited. No grave cuts were visible and the bone fragments seemed to derive from L2. Oyster shells were fairly common in L2 and tended to occur in clusters. Layer 2 was sealed by dark greyish-brown, modern topsoil (L1), which was approximately 500mm thick. In places there were also thin deposits of modern dump/make-up at or near the modern ground surface. The deposits in the trenches were heavily rooted. Several modern brick foundations were encountered in the trenches.