APRA will insist on more consistent rules on the assessment - and reassessment of borrowers over loan serviceability, a measure intended to bolster other rules that, in effect, may restrain credit growth.In guarded language in a letter to all banks and ADIs yesterday, the Australian Prudential Regulation Authority said a revised Prudential Practice Guide for residential mortgage ending explained its updated expectations for sound lending "designed to ensure that the sound lending practices that have been implemented across the industry since late 2014 are maintained and reinforced."In other words, APRA must have detected deviant behaviour, though it qualified: "APRA does not expect these refinements to result in material changes to existing lending practices across the industry as a whole."Its letter clarified some of its concerns.APRA said it "has become aware that some ADIs may have had a practice of permitting changes to the loan type after settlement without conducting a serviceability assessment under the revised loan terms."To address this, APRA directed that "a prudent lender would undertake a new serviceability assessment when making a material change to current or originally approved loan conditions (for example, changing between principal and interest and interest-only repayment terms)."It added that it expects "that a prudent ADI would not rely on longer-term access to 'honeymoon' or discounted introductory rates in assessing ongoing serviceability."APRA also made clear that it won't tolerate any slippage on the "cost of living" measures used across the financial system, having steered the sector away from a historic reliance on poverty line measures over recent years.