Custom software development cost in the UK: the agency build vs the retainer.

The way it's usually sold

First, the pitch you already know.

You describe the business. They listen carefully, go away, and come back with a specification document and a fixed quote. The number has six figures in it, the plan has three phases, and the promise is complete: scope it, spec it, build it, hand it over. Sign here.

None of that is dishonest — for some projects it's exactly right, and we'll tell you which ones below. But for a growing firm that mostly needs its operations to run better, the shape has three faults. All of them are structural.

The sum, from parts you can check

What a conventional build costs, from published day rates.

Most cost guides give you a range and no working. Here's the arithmetic instead. UK suppliers publish their maximum day rates on the government's own cloud marketplace, and the government's trading fund publishes the rates it charges other departments. Price a modest team — two developers, half a delivery manager, half a designer, half a tester; six months; 420 person-days — at three of those published cards:

Strip the team to one full-time developer and a quarter each of the rest and the cheapest card still sums to £134,250. Developers and design are priced at each card's mid level (SFIA 4), testing at SFIA 3, and delivery at the senior delivery level (SFIA 5). All rates exclude VAT. The cards state supplier maximums and awarded work can discount below them; the FCDO card is 2022–23 vintage and includes a security-clearance premium. Every one of those caveats pushes the same way: this is a conservative mid-market sum, not a worst case. Full sources below.

That's why our homepage says an agency can quote £100k–£300k: tested against the published rates, it's polite. And when our pricing page says a traditional agency would price a Build-tier year's output at £60–90k, that money buys 60 to 144 days of team time at these same rates — we think that's modest too, given what a year of Build tier actually ships.

The fourth option

The retainer runs the sum the other way round.

Instead of pricing the whole unknown up front, you pay a flat month: £2,500 (Keep), £3,950 (Build) or £6,500 (Partner) — published in full, three-month minimum, 30 days' notice, and you own the code and data from day one. It starts with a one-week Discovery Sprint (£1,500, credited to your first month), then the smallest module that removes your worst pain — live in weeks — then the next. The spec is never frozen because it's never finished: each month's work is decided when you know the most, not the least.

The reason this arithmetic works deserves naming plainly: AI-assisted development. One senior builder — fifteen-plus years of shipped systems, from child-protection casework to recruitment analytics to a full accountancy platform — uses AI to do the volume work, with every output checked deterministically, and ships at the pace of the team you just priced. That isn't a discount; it's a different cost base. The most recent build ran two years on exactly this model and replaced five subscriptions with one platform that runs ~80 automated jobs a night — the case study shows it in full.

And the market has quietly reached the same conclusion about how ongoing systems should be paid for. The same price guide quoted above says its largest projects — "ongoing systems that power businesses" — involve retainers upwards of £160,000 a year. Ours run £30,000 to £78,000.

Figure 1. Three years of the same ambition, both ways. The leanest credible published-card build (£134,250) buys the launch alone; three years of Build tier (£142,200) includes the build, every change of mind along the way, and the running of it. Keep tier over the same three years is £90,000. Sources in the block below.

Figure 2. The other cost is time-to-anything. A big build's first useful screen arrives at launch; a retainer's arrives in the first month, and the ladder never stops. The method behind the ladder is public — how it works, step by step.

The other side of the ledger

When the agency build is the right call.

A comparison you can trust has to cut both ways, so here's the other side, plainly. Book the agency when:

• You're buying through formal procurement. Regulated sectors and public money often require a fixed scope, a fixed price and a formal delivery contract. That's the job agencies are built for, and they're good at it.
• The scope genuinely is fixed. A well-understood integration, a compliance deliverable, a like-for-like replatform — when the spec truly won't move, a fixed price is exactly the right instrument. Fixed scope is only a fault when the scope was guesswork.
• You're running a £1m+ programme. At that scale you need a bench of specialists, dedicated product owners and a programme office — an in-house team working with an agency, not one senior builder on a retainer.

If that's you, take this page's arithmetic with you — it'll make the procurement sharper. If you're a growing firm whose honest spec would be guesswork, the rest of this page was written for you.

Where this goes next

• The pricing, in full → — because almost nobody in this market publishes prices, and we do.
• Hiring a developer, summed the same way → — about £5,500 a month loaded, and when a hire is genuinely the answer.
• What your manual work costs now — free, no email → — because the sharpest comparison isn't agency vs retainer, it's either vs doing nothing.