The verdict in three sentences
For a maintenance company with 120 engineers, a custom on-call rota app costs between 40,000 and 80,000 GBP (about 46,000 to 92,000 EUR) in 2026 and ships in 3 to 4 months. An off-the-shelf tool at 10-22 GBP per user per month works while your allowance rules stay simple, but it becomes expensive and rigid once your employment contracts, service zones and payroll export come into play. The return on investment comes mainly from allowance errors avoided and the 2 days a month of planning time given back to the operations lead.
Excel, off-the-shelf or custom: the numbers
The on-call rota of a multi-trade maintenance firm (HVAC, electrical, plumbing, access control) combines constraints spreadsheets handle badly: weekly rotations, compensatory rest, night call-outs, Sunday and bank holiday rates, and site-specific qualifications. Here is the 2026 order of magnitude for 120 engineers.
| Criterion | Shared Excel | Off-the-shelf SaaS | Custom app |
|---|---|---|---|
| Upfront cost | 0 GBP | 2,000-5,000 GBP setup | 40,000-80,000 GBP |
| Annual cost (120 users) | 0 GBP | 14,400-31,700 GBP | 4,000-9,000 GBP (hosting + maintenance) |
| Allowance rules from your contracts | Manual, error-prone | Standard rules, hard to adapt | Exact rules of your agreements |
| Payroll export | Re-keying | Generic CSV | Import format of your payroll software |
| Site qualifications | No | Partial | Yes, blocking at assignment |
| Engineer mobile app | No | Yes | Yes (PWA) |
| Code and data ownership | Yes | No | Yes |
Over 5 years, off-the-shelf subscriptions add up to 72,000 to 158,000 GBP for 120 engineers, versus 60,000 to 125,000 GBP for custom, maintenance included. The gap widens as headcount grows, because custom software is not billed per seat.
What the 40,000 to 80,000 GBP budget covers
Scope drives the range. A solid version for 120 engineers includes the following modules.
| Module | Content | Indicative 2026 budget |
|---|---|---|
| On-call rota | Rotations, swaps, manager approval, calendar view | 9,000-16,000 GBP |
| Allowance rules engine | Night, Sunday, bank holiday rates, weekly standby fee, compensatory rest | 8,000-17,000 GBP |
| Engineer mobile app | Rota view, call-out logging, clock-in geolocation | 7,000-14,000 GBP |
| Qualifications and skills | Electrical, Gas Safe, working at height, expiry alerts | 3,500-8,000 GBP |
| Payroll export and dashboards | Monthly pay variables, on-call cost per client | 4,500-10,000 GBP |
| Authentication, roles, audit log | Microsoft 365 SSO, roles, traceability | 3,500-7,000 GBP |
| Testing, rollout, training | Tests over 2 payroll cycles, scheduler training | 4,500-8,000 GBP |
The rules engine is the heart of the project. It turns your employment contracts and any collective agreement into automatically tested rules. That is where allowance errors disappear, and they often represent 1 to 3% of total on-call pay.
Timeline and method: 3 to 4 months without disrupting operations
The project runs in four steps: 2 weeks of scoping with payroll and operations, 6 to 8 weeks of development in increments demonstrated every two weeks, 3 to 4 weeks of acceptance testing in parallel with Excel over a full payroll cycle, then cutover. The parallel run is essential: it proves to payroll that the calculated amounts match or beat the existing ones before the spreadsheet is retired.
UK hosting, UK GDPR compliance for location data (purpose limited to clock-in, short retention) and staff consultation are part of the scoping.
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James, operations director of a facilities maintenance company in London, manages 120 engineers, 18 of them on call each week. His scheduler spends 2 days a month building and fixing the rota, or 24 days a year at about 320 GBP loaded per day: 7,680 GBP. An audit of last year's allowances shows a 2% variance on 290,000 GBP paid, or 5,800 GBP of overpayments and back pay. Add payroll hours spent re-keying variables, estimated at 3,500 GBP a year. Annual gain: about 17,000 GBP. For an app at 58,000 GBP plus 6,500 GBP of yearly maintenance, payback comes in around 5 to 6 years on savings alone, and far sooner against a SaaS at 22 GBP per user (31,700 GBP a year).
FAQ
Why not simply buy an off-the-shelf rota tool?
It makes sense under 30 engineers with simple rules, at 10 to 22 GBP per user per month. Beyond 100 users with specific contract terms, subscriptions exceed 14,000 GBP a year and allowance rules stay approximate.
Can the app export straight to our payroll software?
Yes, we generate the pay variables import file in the format your payroll software expects, with checks before sending. That removes re-keying, which often takes 4 to 6 hours a month in payroll.
How much does maintenance cost after delivery?
Count 4,000 to 9,000 GBP a year depending on service level: hosting, fixes, security updates and minor changes, especially when contracts or pay rates change.
Can we start with a smaller scope?
Yes, a first release covering the rota and allowances at 28,000-34,000 GBP can ship in 8 to 10 weeks. The mobile app and qualifications come in a second release.
Is tracking engineers' location legal?
Yes, if the purpose is specific (clock-in, lone worker safety), staff are informed and retention is short, for example 2 months. Continuous tracking outside call-outs should be avoided.
Let's scope your project. Send us your headcount, on-call rules and payroll software, and we will reply with a scope, a budget between 40,000 and 80,000 GBP and a 3 to 4 month delivery plan. 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.
