The usual UK rule of thumb is that maintaining custom software costs 15–20% of what it cost to build, every year. On a £7,000 build that is £1,050 to £1,400 a year, or £88 to £117 a month. Hosting sits on top: roughly £60 to £400 a month for a modest business application. Our own price is a flat £100 a month, which lands inside that range rather than under it.

Every agency in the country will quote you a build price. Far fewer will tell you what the second year looks like, and that is the number that decides whether a bespoke system was a good idea. This guide covers what maintenance actually costs, what you are buying when you buy it, and the question worth asking before you sign anything.

Where the 15–20% figure comes from

It is a rule of thumb rather than a finding. One UK agency puts it as keeping a system current absorbing "about one-sixth of the work that created it, every year, indefinitely" — and is straight about the fact that the rule survives because it roughly matches observed effort, not because a study established it. Treat it as a sensible planning figure, not a law.

What it is genuinely useful for is sanity-checking a quote. If someone builds you a £40,000 system and then offers to maintain it for £50 a month, one of those two numbers is wrong. And if maintenance is not mentioned in the proposal at all, that is not a saving — it is a cost that has not been named yet.

The three ways UK firms charge for it

Ad-hoc, at £450–£750 a day. You pay for what you use. It looks cheapest and often is, right up until you need somebody urgently and find they are booked for a fortnight. You are buying effort but not availability.

A monthly retainer. A block of hours reserved each month, usually at a lower rate than ad-hoc. Most mid-sized businesses end up here, because the thing they are actually buying is that someone is expecting to hear from them.

A flat managed fee. One monthly figure covering defined maintenance, monitoring and a response time. Predictable, and the model we use.

Hosting is a separate bill, and it is not small

Cloud hosting for a modest business application runs roughly £60 to £400 a month. A busy customer-facing platform can reach £800 to £3,000. Third-party services — payment processing, maps, messaging — usually bill separately again, and they scale with use rather than sitting still.

Ask explicitly whether hosting is inside or outside the monthly figure you have been quoted. It is the most common gap between what a business thinks it agreed and what arrives.

What "maintained" actually looks like

This is the part that is hard to picture from a proposal, so here is a real one. We built a portfolio system for a UK commercial-property firm; it went live on 1 July 2026. In the five weeks after that it took fourteen changes to the database, and that is not a sign anything was missed.

  • Permissions were tightened after launch, because it is only when real people use something that you find where somebody can reach further than they should.
  • One feature was removed entirely. We had built emailed reminders for lease expiries; watching them in use showed they arrived whether or not the thing still needed attention, so they were replaced with a panel on the screen the team already opened daily.
  • A whole second side for acquisitions was added that was not in the original brief.

None of that is emergency repair work. It is what a system in daily use asks for, and it is the honest reason maintenance is not zero. The full account is in the case study.

What our £100 a month covers

Hosting, backups, security, support and small changes. Larger new features are quoted before we build them, so the monthly figure does not quietly become the mechanism by which a project grows. There are no tiers and no per-seat charge, so adding people to your team does not change the bill. That is published on the Proprietary Database page rather than negotiated case by case.

On a £7,000 build, £100 a month is a little over 17% of build cost a year. We are not claiming to undercut the market — we are claiming to be predictable, and to have written down where "small change" stops.

The comparison worth making is against not building at all: TenureBook is £295 a month with nothing to build, and for a lot of firms that is the better answer.

Who owns it, and what if we go away

Our position, published on the systems page: the system belongs to your business, not to us. Hosting and support are optional rather than a condition of having it. Your data exports to Excel whenever you like, and we would rather you left with everything than felt held in place by it.

Being straight about the limits of that: a stated ownership position is not the same as a formal source-code escrow arrangement, and we do not currently operate one. If your system is going to be genuinely business-critical, ask any supplier — us included — to put ownership, access and handover in writing before the work starts rather than after a relationship has soured. A vague answer at that stage is a black box in the making.

What to ask before you sign

  • What does the monthly fee include, and where exactly does "small change" stop?
  • Is hosting inside that figure or billed separately?
  • What is your response time when something breaks, and is it written down?
  • Who owns the code and the data, in writing?
  • If we part company, what do we get handed and in what form?
  • What did you change on your last client's system in the past month?

That final one is the most revealing and the least asked. A supplier who cannot answer it either has no live clients or is not looking after the ones they have.

If you want a straight answer about what your own system would cost to run, tell us what you are thinking of building — including whether you should build it at all.