Within the 18th Edition as far as SPDs and protection against transient overvoltages are concerned there is no need to provide protection for single dwelling units (safe to assume they mean domestic, although there is no definition for a dwelling unit within Part 2 which I think is an oversight). There’s also a mention of protection not being required if the value of the installation and equipment does not justify it. So you would need to weigh up the cost of a SPD against the value of any possibly damaged equipment etc.
Installations where because of the consequences of overvoltage, protection is required…
(i) Hospitals, care homes, health centres etc – where there is a risk of serious injury to, or loss of human life.
(ii) Data centres, heritage buildings – museums, castles, National Trust buildings etc – possible interruption to public services or damage to cultural heritage
(iii) Banks, supermarkets, shopping centres, hotels, offices, factories etc – possible interruption of commercial or industrial activity
(iv) Office blocks, schools, colleges, residential tower blocks (main supply and possibly shared areas but not the individual apartments) – affect a large number of co-located individuals
It also says…
For other cases a risk assessment to 443.5 shall be performed. Which will be highly unlikely as 443.4 covers practically any type of installation you are likely to come across. It’s just the IET covering their backsides.
Plus in order to perform the CRL calculation in 443.5 you would need information that is not available except to the electricity distribution companies (see figure 44.3 and the calculation for risk assessment length ‘Lp’ on page 103).
See BS7671:2018