Boxgrove church comprises an aisled choir, a crossing tower, N and S transepts, and an aisled nave. Only the E bay of the nave is incorporated within the present church; the rest lies in ruins beyond the W wall, which contains the remains of the monastic pulpitum. Due to the presence of the cloister on the N side of the church, only the westernmost 2.5 double bays of the nave had a N aisle. The earliest surviving parts of the building are the plain N and S transepts and the easternmost bay of the nave (c.1120-50). A phased rebuilding programme seems to have begunc.1180-1200, with the 'transitional' central tower, followed by the westernmost five bays of the nave. The rib-vaulted chancel (c.1200-20) is Early English in style, and makes extensive decorative use of Purbeck shafts. Little survives of the monastic buildings. The weathered facade of the chapterhouse probably dates from the mid-12thc. and the ruined guest-house, standing in a field to the NE, is of the 14thc.

In 1086 'the clergy' or 'clerks' (clerici) of Boxgrove (Bosgrave) church held one hide of land, a reference which has been interpreted to mean the church was collegiate. It was given to the Benedictine abbey of Lessay in Normandy in 1105, by Robert de Haye, and was established as a priory c.1117. Initially the monks numbered three, but the number was raised to 15 by the middle of the century. The style of the transept and E nave bay suggests that rebuilding quickly followed the foundation of the priory. Later in the century,c.1170-90, the crossing tower was rebuilt and the nave extended by five bays. The clerestorey of the E nave bay was probably reconstructed around the same time: traces of the earlier round-headed clerestorey opening are revealed in the wall to N and S. The four-bay choir was rebuilt in an Early English style, echoing the Chichester retrochoir,c.1200-20. Later work was insignificant: the addition of a S chapel (now porch) to the E nave bay in the 14thc.; the erection of a sacristy or vestry on the N side of the choir in the 15thc. and the installation of the de la Warr chantry chapel in the S choir arcade in 1532. In 1537, the Priory was suppressed and purchased by Thomas West, Lord de la Warr, for the sum of £136 13s. 4d. It was at this time that the parish moved into the E end of the church, converting the former pulpitum into a W wall and leaving the rest of the nave to fall into ruin. The frater and kitchen were converted into dwellings in the 16thc. and demolished in 1780.

The date of the E bay of the S arcade of the nave, which contains the earliest sculpture in the church, probably lies in the 1130s or 1140s. The multi-scallop capitals are very simple, but one is elaborated with 'V' shapes in a typical High Romanesque manner.

The facade of the chapterhouse also appears to date from the middle of the 12thc., and includes a typical range of High Romanesque motifs such as trefoil and multi-scallop capitals. Its squat proportions suggest an early date, yet the rounded inner orders of the arches are very sophisticated. It seems unlikely that the community would have erected permanent conventual buildings before they had reconstructed the church to their liking, and for that reason this facade can probably be placed after the E nave bay,c.1150.

It has been maintained that the arcading inside the central tower incorporates early 12thc. capitals (eg: Nairn and Pevsner 1965, 115) but that seems uncertain. The crossing incorporates some forms which are considered Romanesque (multi-scallop capitals, animal label stops) and others which are Early English (keeled shafts, pointed arches). Although this work is usually dated toc.1170, parallels in the Chichester Lady Chapel suggest that this should be pushed forward slightly, perhaps toc.1180.

There are few forms in the crossing tower which can be directly compared with the nave extension. The thick, pointed leaves on the upper (and therefore latest) stages of the tower do not sport crockets, while the arcade, clerestorey and vaulting capitals of the nave all do. The impost blocks and string courses of the tower are of standard Norman form, while those in the nave have distinct overhangs. Dogtooth appears in the nave, but not in the tower. Furthermore, there seems to be formal continuity between nave and chancel, which is usually datedc.1220. All of this suggests that the nave (c.1190-1210) followed on from the tower (c.1180), and that the nave workshop then moved on to the chancel (c.1200-20).

The nave extension of Boxgrove respected the style and proportions of the earlier E bay in a manner which echoes the situation at Chichester after 1187, when much of the post-fire rebuilding was inspired by what was already there.

There appears to have been much exchange between the masons of Chichester and Boxgrove in the late 12thc. The closest parallel for the Boxgrove crossing capitals is found in the Chichester Lady Chapel, built before the fire of 1187. The use of Purbeck shafts in the nave of Boxgrove was repeated in the Chichester retrochoir (after 1187), and then again in the Boxgrove chancel. The latter closely resembles Portsmouth Cathedral choir, but their relative chronology is not clear.