In the 15th century permission was granted by Edward IV and confirmed by Richard III for stone to be quarried from the royal forest of Whittlebury to carry out a "Dean substantial reconstruction in the final "Perpendicular" stage of Medieval Gothic". This stone is the distinctive rich brown ironstone, which is quite different from the original limestone.

The nave arcades were re-built and heightened. The south aisle and clerestory windows are also of this period. But the most distinctive addition with its stark exterior lines was the "perpendicular" tower, built in five stories, marked by diminishing stages of the angle buttresses, and rising to a height of 90 feet. All this work took five years, and was completed in 1485.