Yes it should, we should use 1/3 of rating as we feed one RCD from another, typically 1 amp, 0.5 amp, 300 mA, 100 mA, 30 mA, and 10 mA are the common values, but that is often not possible and comply with 30 mA rules, so we do get 30 mA feeding 30 mA, and with AC we always have a little leakage due to capacitance and inductive leaking, so the house RCD will likely always trip first.
So what we should do is use SWA cable which does not need RCD protection and use a high integrity board so garage supply is not protected by house RCD, if not SWA next best is a RCBO in the house, so only that supply lost with garage fault.
But all down to cost, a consumer unit with all RCBO's is far less likely to interrupt the supply to non faulty circuits, and also reduces the back ground leakage per RCD (a RCBO is a MCB and RCD combined) but the consumer unit depending on make used could cost £400 instead of £70 so in domestic it is common to use the cheap option.
Commercial the down time due to loss of supply would be far greater than cost for all RCBO consumer units so they would always fit all RCBO where they can.
So the cure.
1) Find and remove the leakage problem.
2) Check if the CU in house is high integrity or not.
3) Check the type of cable feeding garage.
Decide is it worth the expense to stop same happening again in the future.
Also consider electric car, as if you want to charge an electric car likely a lot of changes will be required, so would be daft to do any upgrade and then have to do it all over again when you get an electric car.
Note:- High Integrity seems an odd name, not a clue why it is called that, but it means it has three neutral bars, this allows normally a couple of circuits either not RCD protected or protected using RCBO's, mothers house had a mini consumer unit for kitchen supplied without going through main RCD's.
So one method if only 2 neutral bus bars is to convert one half of board to all RCBO's, however it depends on age of the board, single width RCBO's have only been in common use for around 20 years, and the early ones were much larger than MCB's so could not be fitted in the older boards, also some firms have got out of Consumer unit manufacturer, and so some parts have gone into short supply. This was not helped by suppliers reducing the cost of the boards that were to be discontinued, so a lot of now obsolete boards were fitted.
