Near the West end of the church stands the beautiful little chapel referred to as either The Chantry Chapel orThe Grammar School.
The style of this architectural gem is of uniformly Perpendicular style in three bays, divided by buttresses, carried up to richly crocketed pinnacle. Each bay has a window of three lights and there are windows of five lights at each end with more elaborate tracery.
Beneath the West window is an arcade of four small windows, probably designed to light an enclosed vestibule within.
It has generally been supposed that it was built as a Chantry, the dual use of such a building not being uncommon inmedieval times.
On the outside please note the scratch dial or Mass clock carved into the stone to the right of the main doorway.
Popular tradition has suggested the Archbishop Henry Chichele, who was born in the town and died in 1443, left a legacy for its construction, but no written evidence has be found of this.
(Chiclele's fine tomb can be seen in Canterbury Cathedral)
It would seem that whoever built the chapel had close connections with the Lancasters.
The emblems around the battlements point to a Lancastrian origin and include the Lancastrian crest of three peacock feathers in a scroll, and may be a tribute to Edward, son of Henry VI, born in 1452.
John of Gaunt, the son of Edward III, had a castle to the North of the church (the site of which is now Castle Fields, a recreation area).
He married Blance of Lancaste, daughter of Henry, fourth Earl (leter Duke) of Lancaster, himself becoming the Duke of Lancaster in 1361 (Blanche having no brothers).
The two (restored) figures on the chapel gables represent John's two sons: on the East Henry Bolingbroke (Henry IV), and on the West Bishop Henry Beaufort, afterwards Crdinal, who was Bishop of the Diocese (then Lincoln) in the 15th century (his mother was Katherine Swynford, John's third wife). Higham Ferrers has been a Royal Duchy since 1399 when Henry Bolingbroke became King Henry IV, bringing the title Duke of Lancaster with him, inherited from his father the previous year.
Thus began the House of Lancaster, one of its emblems being the red rose.
Queen Elizabeth II is the present holder of the title, Duke of Lancaster.
The school may have been amalgamated at the reformation with the Grammar School previously attached to Chichele's College, dissolved in 1538. The surrender of the College to the Crown took place on 18th July 1542.
On 17th April 1543 the College estates (other than the actual College buildings and site) were granted to Robert Dacres. The College is now a National Heritage site situated in College Street just north of the church. Nothing was said about the schoolhouse but a Chapel of Jesus wa mentioned and it is assumed that his was the school building. Robert Dacres was charged to find and maintain "One competent schoolmaster sufficiently learned in the knowledge of grammar----to teach and instruct the children and boys at Higham Ferrers well and diligently--"
Possibly the combined schools found a home in this Chapel, being known as Higham Ferrers Grammar School
The building was used as a school until the final order of closure was issued by the Board of Education on 9th August 1912.
The Master whose picture is hung in the chapel is Mr John Sanderson, known to the boys as "Crack" who kept order by a well aimed book or other suitable missile! He dies in 1883.
The last Master was Mr A.G.C.Vann in whose time the school became co-educational. He died in 1906
After closure of the school the Chapel reverted to church use.
The building has since been extensively restored . In 1914, at the cost of Dr. John Crew, the pitched roof was removed to restore the original roofline. More recently Mr John White generously provided funds to unblock the windows on the North side and restore them to their original condition.
In 1973 the building was repaired out of funds again provided by Mr John White, with the present altar and furnishings also being of his gift. His ashes, and those of his wife Annie, are interred on either side of the altar.

​The Chapel has recently been refurbished (2018) and heating installed so that it can be used in the winter for services without the expense of heating the main church building.
In 1974 the small windows at the West end were reglazed, incorporating fragments of Victorian glass from Walsingham Parish Church, destryed by fire in 1963.
Referring to the Chapel's purpose as a Chantry, no record of its foundation as such appears in the Northamptonshire Chantry Roll, but the theory finds support in the fact that it was a common arrangement of that period to attach some sort of school to a Chantry, and the actual design of the building seems to suggest the same.
On the South side, by the smaller doorway, are ancient stairs, protected by a wooden door, leading possibly to a Rood loft or to enable a watch to be kept on either pupils or the mortal remains of a faithful soul awaiting burial. Who knows?
The roof has an embattled parapet pierced with elegant panelling and, immediately below this, a well moulded cornice ornamented with flowers and leaves.
With the greater part of the wall space occupied by windows giving the building an airy grace, it is always bright within, but on a sunny day the building is flooded with light, causing many to liken it to "a jewel box".
Its actual date of construction is unknown but it was certainly in existence before 1463 when there was an order of the Duchy Court to "amend the gutter next to the School".
Inside the building is seen to be of fine but simple proportions with large perpendicular windows. 
There was probably a western gallery, the space beneath being lit by the small shuttered windows.
The main door opens into a wooden vestibule bearing the date 1636 and decorated with the graffiti of generations of schoolboys.

The Shrine of Our Lady of Walsingham was placed here,
restored and gilded, in memory of Mr Leonard Eagles.

Lord Vaux of Harrowden so admired the building that
he had an exact replica erected at Harrowden Hall in 1905 as his Chapel.