The verdict in three sentences
A SaaS with 1,200 customers maintained on demand, with no guaranteed response time, leaves all incident risk with the product owner. In London in 2026, an application maintenance retainer by hours costs roughly 3,000 to 8,000 GBP a month for 20 to 40 hours covering fixes, dependency updates and monitoring, or about 15 to 20 % of the initial build cost per year at the lower end. The clause that changes everything is critical incident pickup within 2 hours, with service credits if it is missed.
On demand or retainer: what you really pay
On-demand maintenance looks cheap while nothing breaks. In practice, the supplier prioritises contracted clients, security updates pile up, and every incident becomes a negotiation. A retainer flips the logic: reserved capacity, contractual response times and monthly reporting.
| Criterion | On-demand maintenance | Retainer by hours |
|---|---|---|
| Rate | 120 to 200 GBP an hour, no commitment | 110 to 180 GBP an hour averaged, 12-month term |
| Response time | Subject to availability, 1 to 5 days | Critical 2 h, major 8 h, minor 3 business days |
| Dependency updates | If requested | Monthly, scheduled |
| Monitoring and alerts | Rarely included | Included (uptime, errors, performance) |
| Reporting | Invoice only | Monthly report, hours used, incidents |
| Unused hours | Not applicable | Rolled over one month or converted to enhancements |
2026 retainer price grid: London vs Marseille
| Package | Hours a month | London (GBP a month) | Marseille (EUR excl. VAT a month) | Typical commitments |
|---|---|---|---|---|
| Essential | 20 h | 3,000 to 3,800 | 1,800 to 2,100 | Critical within 4 business hours, monthly updates |
| Standard | 30 h | 4,200 to 5,500 | 2,500 to 2,800 | Critical within 2 business hours, 24/7 monitoring |
| Enhanced | 40 h | 6,000 to 8,000 | 3,000 to 3,500 | Critical within 2 h, 7 days, on-call, quarterly review |
| Weekend on-call | Option | 800 to 1,500 | 400 to 800 | Saturday and Sunday response |
| Extra hours | Beyond retainer | 130 to 200 GBP an hour | 85 to 100 EUR an hour | Pre-approval required |
| Initial takeover audit | One-off | 4,000 to 9,000 | 2,500 to 5,000 | Code mapping, tech debt, upgrade plan |
For a SaaS whose initial build cost 250,000 GBP, 15 to 20 % a year means 37,500 to 50,000 GBP, which matches the Essential or Standard package.
Contract terms to insist on
- Incident classification: critical (service down or payments failing), major (key feature degraded), minor.
- Separate response and resolution times, with service credits of 5 to 10 % of the monthly fee per breach.
- Rollover of unused hours for one month at most, or conversion to enhancements.
- Exit plan: up-to-date documentation, access to repositories and infrastructure, handover within 30 days.
- Security: critical patches applied within 72 hours, in line with UK GDPR expectations on security of processing.
Mini case study
Tom, CTO of a 1,200-customer management SaaS in London, pays his supplier on demand. In 2025 he used 310 hours at 160 GBP, or 49,600 GBP, and suffered 3 critical incidents resolved in 26 hours on average. With an average plan of 150 GBP a month, each incident triggered about 8 cancellations, 24 customers lost and 43,200 GBP of annual revenue gone. A Standard retainer at 4,500 GBP a month costs 54,000 GBP a year for 360 hours, with critical incidents resolved within 2 to 4 hours. Even if cancellations only halve, Tom protects about 21,600 GBP of recurring revenue a year for a maintenance budget only 4,400 GBP higher.
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FAQ
How much does a SaaS maintenance retainer cost in London in 2026?
Between 3,000 and 8,000 GBP a month for 20 to 40 hours, depending on guaranteed response times. Weekend on-call adds 800 to 1,500 GBP a month. The same packages from Marseille cost 1,800 to 3,500 EUR excluding VAT.
What share of the initial budget should go to maintenance?
As an order of magnitude, 15 to 20 % of the build cost per year. A 150,000 GBP SaaS therefore needs 22,500 to 30,000 GBP of annual maintenance.
Can a new supplier take over code they did not write?
Yes, after a takeover audit of 4,000 to 9,000 GBP over 1 to 2 weeks. It surfaces tech debt and risks before guaranteed response times are signed.
What happens to unused retainer hours?
Good contracts allow a one-month rollover or conversion into small enhancements. Ask for a monthly report of hours used.
What is the difference between response time and resolution time?
Response is when a developer starts working on the incident, often within 2 hours. Resolution is the return to normal service, usually within 4 to 8 hours for a critical incident.
Let's scope your project. Describe your stack, customer base and incident history, and we will price a maintenance retainer with response commitments and a monthly budget. 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.