That's a great question with no real answer. I see we're in the same general area, I'm on Whidbey Island. It seems PNW boat yards love to estimate low, then only work time and materials and up sell driving the final bill through the roof. I worked alongside the techs at a my haul out and got them to trust me, open up and say things I'm sure management didn't want customers to hear. Such as: "Never work to a bid", "Never give an estimate if you can avoid it, someone else will just estimate lower and you'll loose the job", "Upsell, upsell, upsell". And from previous experiences "Get the boat in now, even if we don't start for months, we don't want anyone else getting the work."
I asked for estimates on numerous work items, not one estimate was reasonably close. The worst being 4X the verbal estimate on labor. When the overrun was approaching 2X the estimate I stopped the work and challenged them. They denied their verbal estimate. I demanded a fresh estimate to finish the work. It went even higher than that. I worked alongside the techs, no one was goofing off or milking the job. Management didn't provide accurate estimates.
I know it doesn't have to be this way. During my career I managed many ship yard projects. They CAN be done to a bid, on time, close to on budget, as specified. With change orders to document and make decisions when "surprises" are found. Of course this means the bid will be high, because it will be honest and give the yard some wiggle room. Estimates on recreational boats seem to be made of dream stuff. The attractive dream of getting it at a good price then becomes the nightmare of reality.
OK, now that my rant is over. Next time I will, understanding I won't get a PNW recreational boat yard to bid the work:
- Write a comprehensive and detailed spec for the work requested. If I'm not competent to do that hire someone who is.
- Require written estimates for each work item including materials, labor and consumables.
- Require a written start and end date. I'd love to apply penalties if they run late but i know no PNW rec yards will hear of that. It will give me a place to argue from when the schedule runs wild.
- Require that any changes to any work item, any additional work be first authorized by me. That the estimate and schedule be amended by a change order.
- Weekly updates on costs and progress.
So, back to your question. How far over? Based on experience with professional yards I expect 25% over. And that 25% should not be due to sloppy estimating but to dealing with surprises found.
100% or more over is not something I will ever go through again.
I think the key is tight specs, close management by you and full understanding the impact on cost and time to add things that seem like a good idea at the time.