Ask three agencies what custom software costs and you will get a rate card, a "it depends", and a discovery workshop invitation. All three are dodging, and it leaves you comparing quotes with no way to judge any of them.
We will not pretend there is one number either, because there genuinely is not. A small internal tool and a multi-system platform are not the same job any more than a garden wall and a house extension are. But the way the cost builds up is the same on every project, and once you can see the parts, quotes stop being mysterious.
Where the money actually goes
Software estimates are mostly labour, and the labour splits into parts that surprise people.
The visible part. The screens, the buttons, the features you asked for. This is what everyone budgets for, and on a well-run project it is often less than half the work.
The invisible part. Login and permissions. Validation on every form. What happens when someone uploads the wrong file, loses connection mid-save, or two people edit the same record. Error handling, backups, an audit trail. None of this appears in a feature list, all of it decides whether the software survives contact with real staff on a busy Tuesday.
The edges. Connecting to your accounts package. Importing ten years of spreadsheet history. The report your accountant needs in one specific format. Edges are where estimates go wrong, because they only surface when someone asks detailed questions about how you work today.
A quote that looks suspiciously cheap has usually priced the visible part and is planning to discover the other two at your expense.
The ranges, honestly
With the caveat that your project is not an average, here is roughly how the UK market shapes up for bespoke work built by senior people.
A small, focused internal tool - one job done properly, a handful of screens, a few users - tends to land in the low thousands to around ten thousand pounds. A serious business application - customer accounts, integrations, reporting, the kind of system a company runs on - typically sits in the tens of thousands. Platforms with many user types, complex rules and mobile apps climb from there.
If a quote sits wildly below these shapes, something is missing from it. Usually the invisible part.
What moves the number most
Integrations. Every system yours must talk to adds real work, and the older or stranger the system, the more. This is the first thing we ask about, because it moves quotes more than any feature.
The number of user types. An admin, a staff member and a customer see three different systems that happen to share a database. Each new role multiplies screens, permissions and testing.
Workflow complexity. "A form goes to a manager for approval" is simple. "Unless it is over five thousand pounds, month end, or the manager is on leave" is where the real cost of your process lives. Software has to handle every branch your business does.
Mobile apps. Adding iOS and Android to a web build typically adds half as much again to double, mostly on store compliance, offline behaviour and device testing. We have written separately about whether you need an app at all - it is the most expensive default assumption in software.
How to make it cheaper without making it worse
Start smaller than you think. The best value project we see is the one that solves the single most painful part of a process first, proves itself in daily use, then grows. It spreads cost, and stage two is always better designed because stage one taught everyone something.
Be generous with detail early. An hour spent walking a developer through how the work really happens - including the awkward exceptions - removes more cost than any negotiation, because uncertainty is priced into every quote as risk.
And insist on a written scope with a fixed price against it. Not because developers are villains, but because a scope forces the detailed questions to happen before the money moves rather than after. If the scope changes later, the difference should be re-quoted in the open, not billed quietly.
The question to ask every developer
Not "how much" but "what would make this more expensive than you have quoted". A good developer answers immediately, in specifics, because they have already thought about it. A bad one says nothing will. One of those quotes is real.
Written by Graeyna
The team that designs, builds and runs the software we write about. Engineers, not marketers.