A large cruciform church comprising chancel with vestries on N (separated from the chancel by a narrow passage contemporary with the chancel), crossing tower, N and S transepts, nave with clerestory, N and S aisles and N and S porches. The church was begun in the mid-12thc. and completed c.1170. The S porch was added in the 14thc., the N porch in the 15thc. and the N vestries in the 19thc. Of clunch, flint and some puddingstone and Roman brick.

Prior to the Conquest the manor of Hemel Hempstead was held of Earl Lewin by two brothers. At the time of DS it was held by the Count of Mortain and the tithe was given to the church of St Mary of Grestain in Normandy. The church appears to have been given to the Abbey of St Bartholomew in Smithfield, London as records show that the canons paid a fine of 200 marks for confirmation of the grant. The canons continued to claim the right to the advowson until 1232 when it was given to the rector of nearby Ashridge.

The 12thc. church was built in what appears to be one continuous campaign, with the chancel and crossing capitals datable to the mid-12thc. and the aisles to the 1160s or 1170s.

The W doorway is compared by Thurlby to the W doorway at St Peter's, Dunstable, in its alternation of carved and moulded orders. Although in general the richness of the decorative scheme of each may be compared, the virtuosity and exuberance of the Dunstable W front carvings is is no way matched by those at Hemel Hempstead. Pevsner suggests that the W doorway has been recut. Details are difficult to read owing to the thick layer of whitewash covering the doorway. It has recently been suggested that the Dunstable W doorway could be as early as 1160 (see Dunstable), which might have implications for the dating of Hemel Hempstead.

Further comparisons may be made between interior features of St Peter, Dunstable and those at Hemel Hempstead. Thurlby points out similarities between the vaulting rib profiles and the form of the capitals in the chancel at Hemel Hempstead and those surviving in the first two bays of the S aisle at Dunstable.

Thurlby also points out similarities between the N nave arcade at Redbourn and the Hemel Hempstead nave arcades. The arch mouldings at Hemel Hempstead, comprising an angle roll followed by a hollow, with billet on the label are paralleled at Redbourne, and some of the scallop capitals are very similar to those at Redbourne.

The fine carving of the crossing capitals echoes that of a series of capitals from the former slype at St Albans Cathedral as well as sculpture on the S transept doorway of the Cathedral. The St Albans carvings are in Totternhoe stone

Pevsner speculates whether the narrow room between the chancel and the vestries was a sacristy.