The verdict in three sentences
For a London SaaS coming out of V1, a maintenance retainer (GBP 3,500 to 10,500 a month for 8 to 20 days) fits once the roadmap is stable and you need contractual fix times. Time and materials at a day rate of GBP 500 to 750 is better while the product is still pivoting and monthly volume swings by more than 50 %. Either way, the contract must lock in reversibility: code, documentation and access handed over unconditionally.
Retainer or time and materials: the real differences
Software maintenance covers three activities: corrective (bugs), preventive (updates, security, dependencies) and evolutive (new features). The billing model shifts risk between you and the vendor.
| Criterion | Monthly retainer | Time and materials |
|---|---|---|
| 2026 price in London | GBP 3,500 to 10,500/month | Day rate GBP 500 to 750 |
| Volume covered | 8 to 20 days per month | As consumed |
| Fix-time commitment (SLA) | Yes, contractual | Rarely |
| Rollover of unused days | Often 1 month max | Not applicable |
| Budget predictability | High | Low |
| Roadmap flexibility | Medium | High |
| Overrun risk | Borne by the vendor | Borne by the client |
A retainer is usually 5 to 15 % cheaper per day than T&M, in exchange for a 6 to 12 month commitment. Beware cheap "unlimited" retainers: they often cover corrective work only.
The service commitments that matter
Price only compares at equal service levels. Here is a realistic grid for a B2B SaaS in 2026.
| Severity | Example | Response | Fix or workaround |
|---|---|---|---|
| Critical (P1) | Platform down, payments failing | 30 min to 1 h | 4 h |
| Major (P2) | Key feature degraded for all users | 2 h | 8 to 24 h |
| Minor (P3) | Bug with a workaround | 1 business day | 48 h to 5 days |
| Enhancement | New feature | Planned into a sprint | Per estimate |
| Out-of-hours on-call | P1 evenings and weekends | 1 h | 4 to 8 h, +15 to 25 % |
On-call is the most underestimated option: for a SaaS sold to customers with a 99.9 % uptime SLA it is essential and adds GBP 500 to 2,200 a month.
Clauses to insist on
| Clause | What it should state |
|---|---|
| Reversibility | Transfer of code, access and documentation within 30 days, no extra fees beyond 3 to 5 days of handover support |
| Intellectual property | Rights assigned as work is delivered, code pushed to your own Git repository |
| Documentation | README, architecture and runbooks updated at every release |
| Reporting | Monthly report: tickets, lead times, days used, technical debt |
| KPIs | Service credits capped at 5 to 10 % of the retainer if SLAs are missed |
| Exit | 1 to 3 months' notice, no commitment beyond 12 months |
Mini case study
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Tom, CEO of a London HR SaaS startup (180 customers), used 9, 16, 11 and 14 days of T&M over the last four months at GBP 650 a day, an average of 12.5 days and GBP 8,125 a month. A 12-day retainer at GBP 7,200 a month, with P1 on-call included at +18 % on the corrective share (about GBP 450), comes to GBP 7,650. Over 12 months he saves about GBP 5,700 and gains a 4 h fix commitment on critical incidents, which he can reuse directly in his own customer contracts.
FAQ
How many maintenance days should we plan after V1?
Allow 15 to 25 % of the initial build effort per year. For a 150-day V1 that is 25 to 35 days a year, or 2 to 3 days a month, plus the enhancement budget.
Can a retainer cover feature work?
Yes, provided you define a day volume and a per-feature estimation process. Days beyond the volume are billed at the contractual day rate, often GBP 550 to 700.
How do we switch vendors safely?
Plan a 2 to 4 week handover with both vendors and a prior code audit costing GBP 2,500 to 7,000. A clear reversibility clause makes the move predictable.
Should we keep a CTO in-house?
A part-time CTO or tech lead is strongly advised to set priorities and accept releases. A fractional CTO costs GBP 1,500 to 4,000 a month in London.
Are security updates included?
They should be part of preventive maintenance, with at least a monthly dependency update. Check the contract, as some vendors bill them on top at 1 to 2 days a month.
Let's scope your project. Share your stack, ticket history and customer SLAs: we will propose a right-sized retainer or T&M setup, GBP 3,500 to 10,500 a month, with a reversibility clause. 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.
