Bishop Lauder was one of Dunkeld’s most active builders, and on 13 April 1457 he began the chapter house on the north side of the eastern limb. This rectangular two-storeyed projection is externally rather plain, being on the side of the building where it would not be particularly visible. Internally it contains a spacious chamber on the ground floor, which is covered by two bays of ribbed vaulting, with a smaller room on the floor above.

Its use as a sacristy as well as a chapter house is confirmed by the provision of a squint at its south-east angle. This was an angled viewing hole which allowed those inside the sacristy to see what was happening at the high altar without stepping out into the presbytery.

A two-storyed building of this type was placed against the eastern limb of several Scottish cathedrals and abbeys to house their ancillary functions. In a cathedral it was usually on the north side, as here, but in an abbey it might be on the south along with the other monastic buildings, as at Arbroath. A variant on the type was a long lean-to range, like those at Dunblane and Fortrose Cathedrals.

 Bishop Lauder’s most prominent addition to the recently completed cathedral was the tower, projecting from the west end of the north aisle. In the grandest medieval buildings there tended to be two western towers together with a tower over the junction of the main body of the building and the crossarms, as at Holyrood Abbey and Aberdeen Cathedral. But many major churches had only one. Sometimes such single towers were free-standing, like those at Cambuskenneth and Lindores Abbey, although probably most were attached to the body of the building, as at Brechin Cathe­dral and Inchmahome Priory.

Dunkeld’s was one of the grandest of the single towers, and when it was built the occasion was also taken to remodel the west front of the nave. The lower part of the front was thickened by the addition of buttresses, between which arches carried a projecting walkway. Above this a very large window was inserted, which had to be set slightly to one side because of the existence of a stair turret leading up to the triforium stage and wall head. Within the gable, and also displaced slightly to one side, was set a delightful small circular window with spiralling tracery patterns.

Only the stubs of the tracery of the main window have survived, though from what there is we can see that it must have been very similar to the exquisite window in the south transept of Linlithgow Parish Church. These two windows, with their complex patterns of interlocking leaf forms, must have been amongst the most ambitious ever to be built in Scotland.

The tower is of four storeys, the lowest of which is covered by ribbed vaulting of the type known as tierceron, from its third order of ribs. At the centre of this vault is a circular hole to allow bells to be hoisted to the belfry stage. The ribs of the vault are decorated with shields bearing Lauder’s arms, and are pierced with rope holes.

The vault retains extensive traces of painted decoration. There are even more extensive traces of decoration within the arches framed by the vaults, where scenes of the judgement of Solomon and of the woman taken in adultery are painted. If these themes seem strange for a church tower, the reason for their choice was that the ground floor stage of the tower was used as the consistory court, in which were heard those cases, including matrimonial matters, which came within the remit of the church.