The nature of the job changes, then so does the bill - but tell them as early as possible.
I'd absorb the costs of the first job myself, both parts. Expecting that one in every ten minor tap-fettlings ends with having to drain down means that I'd stick a few percent extra on in advance, to allow for this. Smoke alarm - my fault, my cost. It's a mistake to combine that aspect with the tap aspect too - they're separate and should be treated as such, annoying though it is.
Also remember how much bad press you receive from a stroppy customer, even when they've received a better deal all round.
As for the second, then I think that's fair to re-estimate when you discover the extra work needed. Worth pointing out on the first estimate though that it's conditional on certain assumptions, such as the joists being OK once uncovered. Point this out well beforehand, even if you're assured they are.
That's his own fault. He should have meggered the installation before making the circuit live, and preferably before giving the quote. Existing installations are too variable for a fixed fee to be given for CU changes.
How many jobs do you over estimate your time? Is it about the same number of hours and jobs you under estimate for? If so then you have got your timing correct and it is time for you to price for a job and not give a price per hour.
You still need to cover your back and charge for the extra parts (such as the gate valve). Your quote must say that these are extras
You must also be prepared to walk away if you find serious defects when you start. There is nothing wrong with telling the customer "I think your electrical system is unsafe and I will not work on it". Redo the quote for the rest of your work excluding the electrics.
Adam
PS I was the soft sod that did a 2 hour return trip to remove a cat from under the floorboards I had nailed down!
If it really is a five min job I would probably do it as a goodwill gesture, but try to agree a time when it would be convenient for me - i.e. when doing other work nearby. For bigger amounts I would charge since you gave an estimate and have a published charging basis. However your "error" was to forget a part of the work, so it would be unfair perhaps to charge the normal premium rate for the first hours of the job, and instead only charge the amount extra that the work would have added should you have continued on the previous occasion.
When doing things like fixing certain computer problems, I often work on the approach of saying we charge £x hour. We can normally fix problems like this in y hours, however it might take much longer. So that you don't have to worry about getting an unexpected bill we will contact you if it looks like the cost is going to exceed z and you can choose what to do.
A phrase that always springs to mind when providing any service is "under promise, and over deliver". So with that in mind, yo could explain the basis of charging - that more work will cost more less will cost less. Then give an estimate - but estimate such that in 90% cases you end up charging less than estimate.
If people are wary of potentially taking on an uncapped liability, then you can also use a "limit of liability" approach; say you think a job is two hours work, you can estimate three hours, and offer a cap at 4 hours
-after which the job price will not increase further. It gives them the security of a fixed price, and the advantage of less cost if the job goes well. You get the protection of having quite a decent latitude cover unforeseen problems at the customers expense without giving them a shock.
Play it by ear in each case and weigh each on its merits. However mitigate the possibility as much as possible in advance by explaining well enough to the customer.
He said he gave a quote, it would be illegal for him to ask for more or to not do the work if it was a quote. If he is going to give quotes he has to do what he says, unless there is a loophole to crawl through.
As for fitting the smoke alarm, the fire brigade supplies and fits them free around here anyway, which gets you good will without you having to do it if you tell the householder.
I believe he gave an estimate not a quote. He also has a published price list that explains the charging procedure (i.e. time and materials based, not fixed price) and pricing. So while not necessarily desirable to charge more than estimated, it is perfectly legitimate.
Its always wise when giving a quote to also state any assumptions that you are basing it on, and perhaps also include a statement that should some of those not be fulfilled extra charges may be due. e.g. "it will cost £x to build the brick wall, this is assuming that the existing foundations are adequate - we will dig an inspection pit on commencing work to establish if this is the case etc."
Two routes you can go down - accept that on some jobs you'll make a lot, on others you might make bugger all, and stick to your quotes. Or go down the route of always charging for what's been done. It just depends on what your customers would prefer. I suspect for one off DIY jobs, they'd prefer a fixed price.
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