No one really knows! Although on my post about Enclosure award Liability I stated 'In some cases the liability may theoretically exist but church has been rebuilt in such a way that the liability' I have since then read a article on what the Tithe Commission thought it seems they considered a chancel was where ever Priest carried out his work. This would suggest that a Chancel for Chancel repair Liability iis not related to architecture since there might only be a nave but if there is an altar that area would be covered by Chancel Repair Liability.
It would seem that the origin of this was that the Lay Rector was responsible for the whole building until the 1923 Ecclesiastical Dilapidations measure and the laity carried out repairs in relief of the rector for the nave but after 1923 the laity took the nave on as an obligation
In other words if someone says there is no liability because there is no chancel they are probably incorrect!
A church rebuilt in a different location under all the various new church acts means the liability transfers to the new building but if the original building continues as a chapel then the liability stays with the original building (1838 New Church Act).
The tithe commission are fairly certain though that a rebuilt chancel cannot increase the liability of the Lay Rector unless they rebuilt it I but otherwise when a chancel is enlarged the Lay Rector is only responsible for the area of the original chancel.
The only time when the liability ends is when a church is made redundant and there is no new church rebuilt in it's place or where there is a long standing tradition of a lay rector not carrying out the repair of the chancel (quite hard to prove).
The Tithe Commission does in part contradict James Derriman and other sources but still there are virtually no precedents and so there is little to go on and the tithe Commission where responsible for Chancel repair liability and Tithes for a long period of time.