Between 1640 and 1652 the north aisle was built by Paul Prédiry and Mathurin Renault, master masons, and the fenestration came from the workshop of Jean Le Bescont and were transported from Landerneau to Saint-Thégonnec and erected in 1651.
The church was enlarged between 1667 and 1669, the apse being moved by several feet, this work being executed by Guillaume Plédran and in 1670 the western part of the nave was raised to accommodate the installation of the organ.
[5] Made from granite quarried from Plounéour-Ménez, this elaborate Renaissance style entrance to the enclos paroissial is attributed to the masons of the nearby Château de Kerjean's workshop.
The two side entrances have stiles to stop animals accessing the enclos and the central arch has iron gates.
[4][6][8] Apart from the Grand Calvaire at Saint-Thégonnec there is a second calvary constructed in 1864 with statues of the Virgin Mary and John the Evangelist by Yann Larhantec.
[4] The baroque pulpit in the Notre-Dame church at St Thégonnec dates to 1683 and was the work of the sculptors François and Guillaume Lerrel, father and son, from Landivisiau.
They carved four female Caryatides representing the Cardinal virtues; Prudence, Temperance, Justice and Fortitude to support the canopy and added panels depicting the Four Evangelists with their attributes around the base.
Many angels are included in the decoration and at the summit a "renommé" stands a foot poised on a globe and playing a trumpet.
The subject of the altarpiece is the adoration of Jesus by the angels and a painting depicts the nativity and in the corners of the altar are statues of the Virgin Mary and Saint Thégonnec.
The lower section dates from 1696 to 1698 and was the work of Jacques Lespaignol with gilding being carried out in 1700 by Gilles Brunel a Morlaix master gilder.
The upper section of the altarpiece has a statue of Saint Louis and an angel holding the hand of a child and in the centre of this upper section there is a second tableau depicting purgatory; Christ, surrounded by angels, receives the souls which Saint Dominic and Catherine of Sienna have rescued from the flames of purgatory.
The ossuary at Saint-Thégonnec stands to the left of the triumphal arch and was built between 1676 and 1682 by the architect Jean Le Bescont (1650-1685).
At the front of the building there is a statue of Saint Pol Aurélien with the dragon he had captured and with two "termes gainé" on either side (also by Jean Le Bescont[8]) and a silver Virgin Mary can be seen on the top of the roof by a small bell-tower.
Inside the ossuary there is a 1685 altarpiece dedicated to Saint Joseph and in the crypt there is a spectacular "mise au Tombeau" by the sculptor Jacques Lespagnol.
[4] This "mise au tombeau" comprises a group of baroque polychromed sculptures carved from oak, depicting Jesus' body being prepared for burial.
In the "mise au tombeau", Lespagnol depicts eleven people expressing their emotions when confronted with death.
Inside the ossuary is a wooden vault ("Voûte en bois") decorated with carvings of angels holding the instruments of the passion and an altarpiece dating to 1685 and dedicated to Saint Joseph.
[4] The main altar of the Notre-Dame church has a tabernacle and is surmounted by statues of both the Virgin Mary and Saint Thégonnec.
[5] On 8 June 1998 the church suffered a bad although accidental fire which devastated the north wing and despite the efforts of the local fire brigade and local people the north aisle of the church was destroyed including the altarpiece "Notre-Dame de Vray Secours" and the nave with the rood screen.
A "poutre de gloire" or rood depicting Christ on the cross, the Virgin Mary and John the Evangelist.
[10] The Église de Saint-Thégonnec, given the classification of a "monument historiques" by the "Commission des Beaux-Arts" in 1886, has undergone many changes since its original construction and now has three naves with seven columns and ten arcades.
In the 18th-century, two doorways were created at the base of the bell-tower to allow processions easy access to the church but this change was subsequently reversed at the insistence of the "Commission des Beaux-Arts".