Fixed Price vs Time and Materials App Development in London (2026)

The verdict in three sentences

Fixed price fits when scope is stable and documented, but you pay a 15 to 25% risk premium built into the price. Time and materials (T&M) is cheaper per day but shifts overrun risk to the buyer. For most SMBs in 2026, the best compromise is capped T&M split into 6 to 8 week lots, with a delivery commitment per lot.

2026 day rates: what a day costs in London

Before choosing a contract model, you need the price of a day. The ranges below are 2026 orders of magnitude seen in the London market (contractors, agencies and consultancies), excluding VAT and expenses.

A 120 person-day project at a weighted average of 680 GBP therefore represents about 81,600 GBP on T&M, before any overrun.

Fixed price, T&M, capped T&M: who carries the risk?

Fixed price remains relevant for a tightly scoped first lot (prototypes, a 6-week MVP). Pure T&M makes sense when the client team can steer a backlog daily. In the UK, when you hire individual contractors on T&M through their own limited company, IR35 off-payroll rules apply: medium and large clients must issue a status determination, and a contractor deemed "inside IR35" costs more once employer contributions are added. Buying from an agency on deliverables usually sidesteps this.

Splitting into 6 to 8 week lots

A 6 to 8 week lot equals 3 or 4 two-week sprints. At the end of each lot, the buyer signs off acceptance, pays for the lot and decides what comes next. This limits financial exposure: you are never committed beyond one lot, about 27,000 to 30,000 GBP per lot for a 120-day project split into 3 lots.

Put in the contract: the cap per lot, the overrun clause (beyond the cap, the vendor finishes at its own cost or at a 50% reduced rate), and acceptance criteria per lot.

Mini case study

Tom, IT procurement lead at a London retail group, must buy a returns management app estimated at 120 person-days. Fixed-price offer: 99,000 GBP. T&M offer: 81,600 GBP estimated, uncapped. He negotiates capped T&M in 3 lots, overall cap 90,000 GBP, overrun billed at 50%. Illustrative example: the project uses 128 days, or 87,040 GBP, under the cap. Saving versus fixed price: 11,960 GBP, about 12%, with the ability to rebalance scope at each lot.

FAQ

What is a typical developer day rate in London in 2026?

For a mid-level developer, expect 500 to 650 GBP as a contractor and 600 to 750 GBP through an agency. Seniors and leads range from 650 to 950 GBP depending on the supplier.

Why is fixed price more expensive than T&M?

The vendor builds in a 15 to 25% risk premium to cover the unknown. If scope is well described, that premium buys peace of mind; if it is vague, you will pay change requests on top.

What is capped time and materials?

It is T&M with a maximum amount per lot or per project. Beyond the cap, the vendor finishes at its own cost or at a reduced rate, sharing the risk while keeping flexibility.

How long should a lot be?

6 to 8 weeks, or 3 to 4 sprints. That limits the commitment to roughly 27,000 to 30,000 GBP per lot for a 120-day project and gives a regular decision point.

Does IR35 affect my choice?

Yes, if you engage individual contractors on T&M, you must assess their IR35 status, and an inside-IR35 determination raises the real cost. Contracting an agency on defined deliverables keeps the risk with the supplier.

Let's scope your project. Tell us about your application: we propose a lot-based plan with a cap, an indicative budget (custom applications from 2,500,000 to 15,000,000+ FCFA, about 3,810 to 22,870+ EUR depending on complexity) and a timeline per lot. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.