There are some readvancable mortgages that are sometimes billed as all-in-one accounts at a few different institutions. The way they work is to basically pull together your mortgage, line of credit, savings, and chequing accounts into one product. The stated benefit is that your emergency fund and cash float from your savings and chequing accounts will be used to pay down the mortgage, but are still available for your use as needed.
All else being equal, this would be a great idea, and indeed recently Nelson at Financial Uproar had a post covering them.
The problem though is that these products usually come with a higher interest rate than you could get on a plain vanilla mortgage, about 0.5-1% more. So does the benefit of having your whole float working on your mortgage balance outweigh the extra cost of the higher interest rate? Like many things, the answer is it depends… but mostly, no.
You can grab a spreadsheet and play along at home with the full calculations if you like, but for once I’m just going to back-of-the-envelope it. There will be some minor effects from the compounding, but not enough to really worry about.
So let’s say you’re a fairly typical recent homeowner: you bought a few years ago, and just recently crossed over into having enough equity to try out this scheme. You’ve got $10k in your emergency fund, and your chequing account balance varies through the month depending on the timing of your bills and payroll deposits, but is on average about $3k. So you could potentially put an additional $13k towards your mortgage with this plan. If you’ve got $400k left on your mortgage, then the amount of interest you’d pay in a year would be $12k at 3%. With the new plan, you’d only be paying interest on $387k, but at the higher rate of 3.5%, which would cost you $13.5k. So the higher interest rate makes it a fair bit more expensive to go with this plan (and any impact of compounding would be balanced by the interest the cash would be generating in a savings account).
If you have more in cash and less owing on your house, it may look better: with $20k in cash and only $100k owing on the mortgage, you’d be paying $3k in interest the traditional way, versus $2.8k for the all-in-one. Still, you need almost absurd amounts of cash compared to the mortgage balance for it to work out.
Plus, with just a little bit of effort on your part, you could in those situations do even better with separate accounts: you can get a regular mortgage at the lower rate, put all of that cash on the mortgage, and open a separate HELOC to get the flexibility to re-withdraw your emergency fund if and when needed. All the money goes on the mortgage at the lower rate — you can even put your chequing account float on the mortgage and use the HELOC for the revolving cash needs (which sounds scary, but is functionally exactly what is happening with the all-in-one products).