John Rumm wrote:

If you are quoting a fixed price for the job, you also have to budget
for potential problems that may not be visible until after the initial
work is done. What happens when a previously non RCD protected circuit
now trips the RCD? How would the customer feel if the electrician turned
round and said, sorry can't get power on to that socket circuit - there
must be a fault in there somewhere, but fixing that is not included in
the price?

The devil, as always, is in the detail. Visual inspections don't give
you the detail. It is often easy enough to spot the installations where
it would be unsafe to do any work without also doing a full or partial
rewire, however the systems that "look ok" but have hidden problems can
take far more fixing.

I think the above is the crux of it - attaching a new consumer unit to
an old installation is highly likely to highlight hitherto-undetected
faults which mucxt be traced and rectified before the new CU can be
wired up.

Personally, I've twice in the past undertaken full rewires of properties
I've been renovating; in both cases the plan was to start by replacing
the CU and hooking it up to the old wiring, which I would then replace
piecemeal. However in *both* cases the RCDs tripped immediately
(despite both properties having up-and-running electrics previously). I
judged it wasn't worthwhile tracing the faults and just wired up one new
double socket to provide temporary power while I did all the rewiring.

Admittedly the wiring was in a pretty poor state (which was why I'd
planned on replacing it) - hopefully yours isn't so bad!

David