Before answering this, it is worth saying what the evidence base is, because most pages answering it have none.
There is no dataset of contractor mistakes
Search this question and you will find lists: underquoting, poor scheduling, weak communication, taking on too much work. None of it is measured. It comes from trade blogs restating one another, and no authoritative body publishes anything on what contractors get wrong from the contractor's side.
What does exist is complaint data: a national record of what goes wrong from the customer's side. That is a proxy rather than the same thing, and it is the only quantified evidence available, so it is what this answer is built on.
What the complaint data shows
Citizens Advice recorded 36,534 home maintenance and improvement complaints in the year from 1 July 2024 to 30 June 2025, around 700 every week.
- 19,281 (52.8%) concerned low-quality services
- 5,230 (14.3%) concerned scams or rogue traders
That ratio is the finding. Poor-quality work by real traders is roughly four times more common than the cowboy who takes the money and disappears. The commonest failure is competent-but-sloppy rather than villainous. That is the opposite of what most consumer advice prepares people for.
The issue-type breakdown points the same way: failure or delay in providing the service accounted for 2,546 complaints (7%), defective goods 2,299 (6.3%) and breach of contract 2,041 (5.6%).
Which trades generate the most complaints
| Rank | Category | Complaints | Share | |---|---|---|---| | 1 | Roofing, roof sealing, chimney repairs | 8,126 | 22.2% | | 2 | Major renovations: lofts, conversions, extensions | 4,365 | 11.9% | | 3 | Window frames and doors | 3,856 | 10.6% | | 4 | Plumbing services | 2,629 | 7.2% | | 5 | Fitted kitchens | 2,594 | 7.1% |
Roofing alone is over a fifth of all home improvement complaints, and the top two categories together are a third. Both are jobs where the work is hard for a homeowner to inspect, expensive to redo, and where the defect surfaces long after the money has gone.
What this changes about how you hire
If the dominant risk were rogue traders, the right response would be a fraud screen: check the doorstep approach, the cash demand, the missing address. Those checks are still worth doing, and they catch the 14.3%.
But they do nothing about the 52.8%. Catching poor-quality work needs a different set of steps, and they are all unglamorous:
- Three written quotes, none of them the first one you were given. The FMB advises exactly this, and notes the cheapest is not automatically the best.
- A written contract covering the standard the work must reach, how defects will be put right and who is responsible, and a snagging agreement.
- References you can visit, for jobs two or three years old rather than two months old. Quality shows up in weather, not on handover day.
- Staged payments tied to verifiable stages, so problems can be corrected before the final payment. That is Citizens Advice's reasoning for recommending them.
- Evidence as you go: receipts, photographs and dated notes. Citizens Advice lists this among its four core tips, and it is what makes any later remedy usable.
The mistakes that are evidenced
Turning the same sources around, the failures with a documented basis are mostly about process rather than craft:
- Taking the first quote, or choosing on price alone
- Working without a written contract, so nothing fixes the standard or the programme
- Leaving VAT unclarified, so two quotes were never comparable
- Taking too much money upfront, or paying it in cash
- No agreed schedule of works, and no pre-agreed rate of liquidated damages if the job overruns
- No snagging agreement, so the last 5% of the job never gets defined
- No contingency in the budget, where the FMB suggests at least 10%
- No agreement on who orders materials
Every one of those is a failure of paperwork rather than of skill, and every one is fixable before anybody starts.
The honest limit of this answer
This is complaint-side data, so it tells you what homeowners reported, not what builders did. A trade with more complaints may have more jobs, more expensive jobs, or work that is harder to get right. Roofing plausibly has all three.
What it does establish, and what almost nothing else in this area establishes, is the proportion. If you are preparing for one failure mode, prepare for the ordinary one.
The unglamorous checks are the ones that catch the common failure.