You priced the parts, added them up, and the finished machine on the shop page costs more than your list. The gap looks like the shop taking a margin for screwing components together — work you could do yourself in an afternoon. Some of that gap is exactly that. The rest of it is a specific thing you cannot assemble at home, and it is worth knowing which part of the difference is which before deciding.
What is inside the difference
Comparable specifications in the UK market typically put a prebuilt £50 to £150 above the parts list, though the gap moves with component pricing and occasionally inverts, because a builder buying trays of processors does not pay the price you see on a retail page.
Three items account for most of it, and only one of them is labour.
The operating system. A machine sold with Windows includes a licence you would otherwise buy, at £100 to £150 retail. That single line can be most of the gap, and it is the reason a like-for-like comparison that leaves the licence out of the parts list is not a comparison at all.
Assembly and test. A first-time builder spends an afternoon; a shop spends a fraction of that and then runs the machine under load long enough to catch the components that fail early. The testing is the part with value, not the screwing together.
A warranty on the whole machine. This is the item with no home equivalent, and the rest of this piece is about what it is actually worth.
How often does anything fail
The useful numbers here come from builders who publish their own failure data rather than from manufacturers. A large system integrator's reliability reporting separates failures found in the workshop before shipping from failures that happen at the customer, and the field figures are the interesting ones.
Motherboards run around 1 per cent field failure. Well-regarded memory came in at 0.15 per cent and one popular SSD line at 0.16 per cent. Processors vary by generation and vendor rather than by category: one Intel generation showed a 5.28 per cent workshop failure rate, while comparable AMD workstation parts showed field failures of 0.80 per cent against Intel's 1.32 per cent.
Take those as roughly half to one per cent per component, for the six or seven parts in a typical build. The probability that every part survives is the product of the individual survival rates, so the probability that something fails is what remains: with six components at 1 per cent each, about 6 per cent. At half a per cent each, about 3 per cent.
That is our arithmetic rather than anybody's published figure, and it is the number that matters. The chance that any specific part fails is small enough to shrug at. The chance that one of the seven does is small enough to gamble on and large enough to plan for — roughly one machine in twenty.
The part you cannot assemble at home
Here is what that one machine in twenty costs in each of the two paths.
With a prebuilt, there is one telephone number. You do not have to know which component failed — diagnosis is the seller's problem, and so is the possibility that the fault is an interaction rather than a part. The machine goes back, or an engineer comes to it, and the specification you bought is what has to work when it returns.
With a self-build, you own the diagnosis. That means isolating the fault to a component, then dealing with whichever retailer sold that component, under that retailer's returns process, on that retailer's timetable. Seven parts can mean seven different sellers, seven different return windows and, in the worst case, a fault that no single seller accepts because each part tests fine on its own bench.
British consumer law is more generous than most people realise here, and it applies to both paths. Under the Consumer Rights Act 2015 the retailer — not the manufacturer — carries the obligation, and a claim can be brought for up to six years in England, Wales and Northern Ireland, or five in Scotland. Manufacturer warranties sit on top of that and cannot reduce it.
The catch is the burden of proof. After the first six months, it shifts to you: you must show the goods were faulty when sold. For a whole machine bought as one product, that is one argument with one seller. For a graphics card bought separately eighteen months ago, it is your word against a bench test, and the practical difficulty is exactly the reason the single-seller path has value.
When the premium is not buying you that
None of this defends every prebuilt, and the failure modes are consistent enough to check for by name.
- An unnamed power supply. If the listing says "700 W" without a manufacturer and model, that is the component the margin came out of, and it is the one that can take others with it when it fails.
- An unnamed motherboard. "B-series motherboard" is not a specification. It hides the power delivery, the socket's upgrade path and the number of storage slots you will find when you open it.
- A headline part carried by a weak platform. A current graphics card in a machine with four sticks of slow memory and a hard drive is a build shaped by a price target rather than a use.
- Proprietary shapes. A non-standard power supply or motherboard means the machine cannot be upgraded with ordinary parts — you are not buying a PC so much as an appliance in the shape of one.
The recommendation, stated plainly
Buy the prebuilt if you want one address for problems and no interest in diagnosing them — but only from a seller who names every component, because a listing that hides the power supply and the motherboard is hiding where the money went.
Build it yourself if you are prepared to own the diagnosis when one part in twenty machines fails, and if the parts list you compared included the operating-system licence. The saving is real, and it is smaller than the raw subtraction suggests.
What you should not do is pay a prebuilt premium for a machine whose components are unnamed. That is the one case where the gap buys neither the licence, nor the testing, nor a warranty worth having — because the parts you cannot see are the ones the warranty will be tested on.
The check that takes a minute
Take the listing and write down every component with its manufacturer and model. If the power supply and the motherboard are not both named, that is your answer without any arithmetic. If they are, price that exact list at one retailer, add £100 for the licence, and compare. What remains is what you are paying for assembly, testing and a single point of contact — and you can now decide whether that is worth it to you, rather than guessing at what the gap contains.
How this was put together
Five independent sources sit under the figures above: UK market pricing that puts comparable prebuilts £50 to £150 above a parts list; retail pricing of a Windows licence at £100 to £150; a system integrator's published reliability reporting, separating workshop from field failures and giving roughly 1 per cent for motherboards, 0.15 to 0.16 per cent for memory and one SSD line, 5.28 per cent workshop failures for one Intel generation, and field failures of 0.80 against 1.32 per cent for two processor families; the Consumer Rights Act 2015, under which the retailer carries the obligation and claims run to six years in England, Wales and Northern Ireland and five in Scotland; and the six-month point at which the burden of proof shifts to the buyer.
The derived figure is ours: the roughly 3 to 6 per cent chance that at least one component in a six-part build fails in the field, computed from the per-component rates above, and the reading that this — rather than the cost of assembly — is what the warranty on the whole machine is actually priced against.
