EIM23465 - Car and van benefit: pooled cars and vans: meaning of not normally kept overnight
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Sections 167(3)(e) and 168(3)(e) ITEPA 2003
You can accept that a car is not normally kept overnight at employees’ homes if the total number of nights on which it is taken home by employees, for whatever reason, is less than 60% of the total number of nights in the period under review.
This 60% test is no more than a rule of thumb that you can apply in order to reach a reasonable settlement in most cases. It is also mentioned in Booklet 480 (Expenses and Benefits - a tax guide). However, if it should be necessary to consider the interpretation of either Section 167(3)(e) ITEPA 2003 (cars) or Section 168(3)(e) ITEPA 2003 (vans) in an appeal before the First-tier Tribunal, rely on the statutory definition rather than this rule of thumb.
Note that if a car or van is taken home often enough to approach the 60% limit, though without breaking it, it is unlikely that all the home to work journeys will satisfy the merely incidental test in EIM23455.