St John the Baptist's Church is a typical example of the simple two-cell (nave and chancel) layout found at many pre-Norman Conquest churches in Sussex.
 The plan consists of a square-ended chancel, a much taller nave, a porch on the north side, a vestry on the south side and a west-end belfry of timber and shingles.
  The "tall, thin walls" of the nave give the church a "heartfelt piety".
 The church is built of flint with some sandstone dressings and quoins, covered with cement in places.
 The roof is laid with a mixture of red tiles and Horsham Stone slabs.

The "bold", "impressive", "solid and powerful" chancel arch is the principal structural feature inside.
 It dates from the 11th century and is flanked by a pair of arched recesses, one of which retains some original plasterwork.
 These may have served as squints originally.
 The structure dominates the nave through its sheer height, the use of massive square stone blocks with a smooth, plain finish, and the three 10-inch (25 cm) moulded shafts on each side.
 The jambs terminate in bulky chamfered imposts.
 The arch has been compared to that of another Grade I-listed Anglo-Saxon church in West Sussex—the slightly older St Nicholas' Church at Worth.
 Its three bells date from the early 15th century (by Richard Hille), the 1470s (Henry Jurdan) and 1713 (Samuel Knight).
 Hille owned a foundry in London; his bells are found at several other churches throughout Sussex.

The chancel, renewed in the 19th century, measures 19.5 by 13.25 feet (5.94 m × 4.04 m).
 The east window is a three-light lancet with mullions, set below a segmental-arched hood mould.
 There are two narrow lancets in the south and north walls as well, and a blocked window of Anglo-Saxon origin on the latter.
 The walls are just over 2 feet (0.61 m) thick.

The nave formerly had a pair of porticus-style side chapels, but little trace of these remains—although blocked windows and fragments of archways and gabled roofs have been visible since 1918, when they were excavated.
 The nave's dimensions are 43 by 22.5 feet (13.1 m × 6.9 m), and the walls are about 2.5 feet (0.76 m) thick.
 The Norman doorway, reset on the north side in its 15th-century porch, have jambs with five stones of irregular length and a five-stone arch.
 The belfry, at the west end of the nave, sits on top of the roof with no structural link to the inside of the building.
 It has a shallow pyramid-shaped roof.

Other than the wall paintings, the interior is plain.
 The ceiling has panelling and simple timberwork, and walls with no murals are plastered.
 There are some 18th-century Eucharistic objects and a 17-inch (43 cm) brass memorial to Richard Idon, a parson, who died in 1523.
 He is shown holding a Communion wafer and chalice and clad in vestments.
 Another brass, slightly older (1508) but consisting solely of an inscription to Thomas a Wode, is hidden under a carpet.
St John the Baptist's Church is a typical example of the simple two-cell (nave and chancel) layout found at many pre-Norman Conquest churches in Sussex.  The plan consists of a square-ended chancel, a much taller nave, a porch on the north side, a vestry on the south side and a west-end belfry of timber and shingles.   The "tall, thin walls" of the nave give the church a "heartfelt piety".  The church is built of flint with some sandstone dressings and quoins, covered with cement in places.  The roof is laid with a mixture of red tiles and Horsham Stone slabs.
The "bold", "impressive", "solid and powerful" chancel arch is the principal structural feature inside.  It dates from the 11th century and is flanked by a pair of arched recesses, one of which retains some original plasterwork.  These may have served as squints originally.  The structure dominates the nave through its sheer height, the use of massive square stone blocks with a smooth, plain finish, and the three 10-inch (25 cm) moulded shafts on each side.  The jambs terminate in bulky chamfered imposts.  The arch has been compared to that of another Grade I-listed Anglo-Saxon church in West Sussex—the slightly older St Nicholas' Church at Worth.  Its three bells date from the early 15th century (by Richard Hille), the 1470s (Henry Jurdan) and 1713 (Samuel Knight).  Hille owned a foundry in London; his bells are found at several other churches throughout Sussex.
The chancel, renewed in the 19th century, measures 19.5 by 13.25 feet (5.94 m × 4.04 m).  The east window is a three-light lancet with mullions, set below a segmental-arched hood mould.  There are two narrow lancets in the south and north walls as well, and a blocked window of Anglo-Saxon origin on the latter.  The walls are just over 2 feet (0.61 m) thick.
The nave formerly had a pair of porticus-style side chapels, but little trace of these remains—although blocked windows and fragments of archways and gabled roofs have been visible since 1918, when they were excavated.  The nave's dimensions are 43 by 22.5 feet (13.1 m × 6.9 m), and the walls are about 2.5 feet (0.76 m) thick.  The Norman doorway, reset on the north side in its 15th-century porch, have jambs with five stones of irregular length and a five-stone arch.  The belfry, at the west end of the nave, sits on top of the roof with no structural link to the inside of the building.  It has a shallow pyramid-shaped roof.
Other than the wall paintings, the interior is plain.  The ceiling has panelling and simple timberwork, and walls with no murals are plastered.  There are some 18th-century Eucharistic objects and a 17-inch (43 cm) brass memorial to Richard Idon, a parson, who died in 1523.  He is shown holding a Communion wafer and chalice and clad in vestments.  Another brass, slightly older (1508) but consisting solely of an inscription to Thomas a Wode, is hidden under a carpet.
St John the Baptist's Church, Clayton