This SS has not yet been added to the PRA Rulebook. It is available on the Bank of England website, here.

https://www.bankofengland.co.uk/prudential-regulation/publication/2016/underwriting-standards-for-buy-to-let-mortgage-contracts-ss

Related links

PS28/16 - Underwriting standards for buy-to-let mortgage contracts https://www.bankofengland.co.uk/prudential-regulation/publication/2016/underwriting-standards-for-buy-to-let-mortgage-contracts