The east end, facing the piazza, is now faced in stone, with a massive portico, its boldly-projecting pediment supported by two columns and two piers.
There were originally three doorways behind the portico; the middle one, which survives, was built as a false door as the interior wall behind it is occupied by the altar.
The other two were blocked up in the 19th century, when the chancel floor was raised.
The main entrance to the church is through the plainer west front, which has a pediment, but no portico.
William Prynne, writing in 1638 said that it was originally intended to have the altar at the west end, but pressure from the church hierarchy led to the imposition of the traditional orientation.

The earliest existing detailed description, dating from 1708, says that the exterior was not of bare brick, but rendered with stucco.
In 1789 it was decided to case the walls in Portland stone as part of a major programme of renovation, which Thomas Hardwick was chosen to supervise.
At the same time the tiled roof was replaced with slate, the dormer windows, added in the 1640s, were removed, and the archways flanking the church, originally of stuccoed brick, were replaced with stone replicas.
 When Hardwick's stone facing was removed from the church in 1888, it was found to be a thin covering less than three inches thick, poorly bonded to the brick.
The building was then reclad in the present unrendered red brick.

There were originally six or seven steps leading up to the portico, but these disappeared as the level of the Piazza was raised gradually over the years.
By 1823 there were only two steps visible, and none by 1887.
The arches at the side of the portico were substantially widened and raised during a restoration of 1878–82 by Henry Clutton, The 9th Duke of Bedford's architect.
 Clutton also removed the bell-turret over the western pediment.

The interior is a single space, undivided by piers or columns.
The eastern third was originally marked out as a chancel by means of the floor being raised by one step.
The level was raised further during alterations by William Butterfield in 1871–72.
The church was built without galleries, but they were soon added on three sides.
Hardwick included them in his rebuilding, and the western one remains today.

To the sides of the portico there were two elaborate pedimented gateways to the churchyard that were removed, then replaced, continuing the "Tuscan" style.
 Plans and historical images show a variety of relatively small structures rising from the roof, some shown housing bells.
The roof is now completely bare, and two bells can be seen in recesses cut high on the western facade.

The portico was a great inspiration for Roger Morris (1695 - 1749) when designing the stable block for Althorp House, Northamptonshire, which is believed to have been built between 1732 and 1733.
In the Palladian style and constructed of local ironstone, the east and north sides of the stables feature the deep porticos, both of which face the house.
Many architectural historians have expressed the view that the stables are of more architectural importance than the house itself, which is rather more plain in comparison as a result of many centuries of alterations, including a major exterior renovation by Henry Holland (1745 - 1806) in the 1790s.
The east end, facing the piazza, is now faced in stone, with a massive portico, its boldly-projecting pediment supported by two columns and two piers. There were originally three doorways behind the portico; the middle one, which survives, was built as a false door as the interior wall behind it is occupied by the altar. The other two were blocked up in the 19th century, when the chancel floor was raised. The main entrance to the church is through the plainer west front, which has a pediment, but no portico. William Prynne, writing in 1638 said that it was originally intended to have the altar at the west end, but pressure from the church hierarchy led to the imposition of the traditional orientation.
The earliest existing detailed description, dating from 1708, says that the exterior was not of bare brick, but rendered with stucco. In 1789 it was decided to case the walls in Portland stone as part of a major programme of renovation, which Thomas Hardwick was chosen to supervise. At the same time the tiled roof was replaced with slate, the dormer windows, added in the 1640s, were removed, and the archways flanking the church, originally of stuccoed brick, were replaced with stone replicas.  When Hardwick's stone facing was removed from the church in 1888, it was found to be a thin covering less than three inches thick, poorly bonded to the brick. The building was then reclad in the present unrendered red brick.
There were originally six or seven steps leading up to the portico, but these disappeared as the level of the Piazza was raised gradually over the years. By 1823 there were only two steps visible, and none by 1887. The arches at the side of the portico were substantially widened and raised during a restoration of 1878–82 by Henry Clutton, The 9th Duke of Bedford's architect.  Clutton also removed the bell-turret over the western pediment.
The interior is a single space, undivided by piers or columns. The eastern third was originally marked out as a chancel by means of the floor being raised by one step. The level was raised further during alterations by William Butterfield in 1871–72. The church was built without galleries, but they were soon added on three sides. Hardwick included them in his rebuilding, and the western one remains today.
To the sides of the portico there were two elaborate pedimented gateways to the churchyard that were removed, then replaced, continuing the "Tuscan" style.  Plans and historical images show a variety of relatively small structures rising from the roof, some shown housing bells. The roof is now completely bare, and two bells can be seen in recesses cut high on the western facade.
The portico was a great inspiration for Roger Morris (1695 - 1749) when designing the stable block for Althorp House, Northamptonshire, which is believed to have been built between 1732 and 1733. In the Palladian style and constructed of local ironstone, the east and north sides of the stables feature the deep porticos, both of which face the house. Many architectural historians have expressed the view that the stables are of more architectural importance than the house itself, which is rather more plain in comparison as a result of many centuries of alterations, including a major exterior renovation by Henry Holland (1745 - 1806) in the 1790s.
St Paul's, Covent Garden