The dark art of measuring residential mortgage risk

Residential mortgages are one of the seemingly more plain vanilla forms of bank lending. Notwithstanding, comparing capital requirements applied to this category of lending across different types of banks can be surprisingly complicated and is much misunderstood. I have touched on different aspects of this challenge in a number of mortgage risk weight “fact check” posts (see here and here), focussing for the most part on the comparison of “standardised” capital requirements compared to those applied to banks operating under the “internal rating based” (IRB) approach.

A discussion paper (“A more flexible and resilient capital framework for ADIs”, 8 December 2020) released by the Australian Prudential Regulation Authority (APRA) offers a good summary (see p27 “Box 2”) of the differences in capital requirements not captured by simplistic comparisons of risk weights. However, one of the surprises in the discussion paper was that APRA chose not to address one of these differences by aligning the credit conversion factors applied to off-balance sheet (non-revolving) residential mortgage exposures.

Understanding why APRA chose to maintain a different treatment of CCFs across the two approaches offers some insights into differences in the way that the two approaches recognise and measure the underlying risks.

Before proceeding we need to include a short primer on “off-balance exposures” and “CCFs”. Feel free to skip ahead if you already understand these concepts.

  • Off-balance sheet exposures are the difference between the maximum amount a bank has agreed to lend and the actual amount borrowed at any point in time.
  • The CCF is the bank’s estimate of how much of these undrawn limits will in fact have been called on (converted to an on balance sheet exposure) in the event a borrower defaults.
  • In the case of “non-revolving” residential mortgages, these off-balance sheet exposures typically arise because borrowers have got ahead of (“pre-paid”) their contractual loan repayments.

APRA noted that the credit conversion factor (CCF) currently applied to off-balance sheet exposures was much higher for IRB banks than for standardised, thereby partially offsetting the lower risk weights applied under the IRB approach. It had been expected that APRA would address this inconsistency by applying a 100% CCF under both approaches.

Contrary to this expectation, APRA has proposed to revise the CCFs applying to (non-revolving) off-balance sheet residential mortgage exposures as follows:


Standardised 0-50%

IRB 100%



100% (unchanged)

The interesting nuance here is that APRA is not saying that standardised banks are likely to experience a lower percentage drawdown of credit limits in the event a borrower defaults. In the “Response to Submissions” that accompanied the Discussion Paper, APRA noted that “Borrowers do not typically enter default until they have fully drawn down on their available limit, including any prepayments ahead of their scheduled balance.

However, APRA also noted that loans with material levels of prepayment are also likely to be lower risk based on the demonstrated greater capacity to service and repay the loan.

Under the IRB approach, the greater capacity to repay the loan is generally recognised through a lower PD estimate which the IRB formula translates into lower risk weights reflecting the lower risk. In the absence of some equivalent risk recognition mechanism in the standardised approach, APRA is proposing to use a concessional CCF treatment to reflect the lower risk of loans with material levels of prepayment. It notes that the concessional CCF treatment will also contribute to ensuring the difference in residential mortgage capital requirements between the standardised and IRB approaches remains appropriate.

Summing up:

  • Looked at in isolation, 100% is arguably the “right” value for the CCF to apply to off-balance sheet exposures for a non-revolving residential mortgage irrespective of whether it is being measured under the standardised or IRB approach
  • But a “concessional” CCF is a mechanism (fudge?) that allows the standardised approach to reflect the lower risk associated with loans with material levels of prepayment

Tony – From the Outside

Author: From the Outside

After working in the Australian banking system for close to four decades, I am taking some time out to write and reflect on what I have learned. My primary area of expertise is bank capital management but this blog aims to offer a bank insider's outside perspective on banking, capital, economics, finance and risk.

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