The verdict in three sentences
The maintenance retainer for a custom web app is budgeted at 15 to 25 % of build cost per year, i.e. a common 900 to 3,200 GBP/month depending on SLA. This budget is not optional: without it, dependencies age, vulnerabilities accumulate and the cost of catching up explodes. A good retainer covers corrective, evolutive work, security, dependencies and monitoring with clear response-time commitments.
What a maintenance retainer covers
Not all retainers are equal. Here are the components and their typical weight in a 2026 contract.
| Component | Content | Share of retainer |
|---|---|---|
| Corrective maintenance | Bugs, incidents | 25 % |
| Evolutive maintenance | Small changes, tweaks | 30 % |
| Security & dependencies | Patches, library updates | 20 % |
| Monitoring & supervision | Alerts, uptime, logs | 10 % |
| Hosting & backups | Infra, tested backups | 10 % |
| Support & advisory | Hotline, guidance | 5 % |
Retainer grid by SLA level
Price tracks the commitment level (response times, availability). Order of magnitude for 2026.
| SLA level | Response time | Target uptime | Retainer GBP/month |
|---|---|---|---|
| Basic | 2-3 business days | 99.0 % | 900 - 1,300 |
| Standard | 1 business day | 99.5 % | 1,300 - 2,200 |
| Premium | 4 business hours | 99.9 % | 2,200 - 3,200 |
| Critical 24/7 | 1 hour, on-call | 99.95 % | 3,200 - 6,500+ |
Managed hosting (2,000-8,000 GBP/year) is sometimes included, sometimes billed separately: check the scope.
Mini case study
Claire, CFO of a manufacturing SME in London, commissioned a production-management app for 95,000 GBP. She budgets the retainer at 18 % of build cost, i.e. 17,100 GBP/year (~1,425 GBP/month, standard SLA). In year one, the retainer absorbs 2 blocking incidents (resolved in < 1 day), 9 minor changes and security updates. Without a retainer, a single blocking incident during month-end close would have cost, in lost production and emergency work, more than the entire annual retainer.
FAQ
Need a professional website?
Kolonell builds websites that attract clients, optimized for the Sénégalese market. Free quote in 2 minutes.
Why 15-25 % of build cost?
It is the observed ratio to keep an app healthy: fixes, security, small changes and dependency upkeep. Below that, technical debt accumulates.
Does the retainer include major changes?
No: it covers corrective work and small changes. A rebuild or new module is quoted separately.
Can you pause the retainer to save money?
Not advisable: dependencies and security flaws do not wait. Resuming after 12 months of neglect often costs more than a year of retainer.
Who manages hosting?
Depends on the contract: either included in the retainer or billed separately (2,000-8,000 GBP/year). Clarify at signing.
Build vs buy long term?
Over 5 years, custom + retainer often stays competitive against a SaaS's recurring licenses, especially at high user counts.
Let's scope your project. Tell us your app's build cost and target SLA, and we'll frame a fairly priced maintenance retainer. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
