In-House, Agency, or Contractor? How UK Businesses Should Build Software Teams in 2026

There are three ways to get software built: hire engineers directly, bring in freelance contractors, or engage an agency. Most businesses default to whichever one they used last time, without weighing the three against the specific project in front of them. Each model has a real, different cost and risk profile — worth understanding before you commit budget.

The three models, compared

In-house hire

• Cost: UK salary £45,000-£90,000+ depending on seniority, plus roughly 20-30% on top for employer National Insurance, pension contributions, benefits and recruitment cost.
• Speed to start: Slowest — realistic hiring timelines for a strong engineer currently run 6-12 weeks from job posting to start date, longer for senior or specialist roles.
• Continuity: Best, in theory — but a single hire is a single point of failure. If they leave, you lose institutional knowledge overnight.
• Best for: A genuinely ongoing, multi-year product with steady headcount need, and a business large enough to support proper engineering management and career progression.

Freelance contractor

• Cost: £400-£900+ per day depending on seniority and specialism, no employer NI or pension, but no guaranteed availability either.
• Speed to start: Fast for well-networked buyers, but quality varies enormously and vetting takes real time if you are doing it yourself.
• Continuity: Weakest — contractors are, by definition, temporary, and a single contractor leaving mid-project is a real delivery risk with no built-in cover.
• IR35 exposure: Since April 2021, medium and large private-sector businesses are responsible for determining the IR35 (off-payroll working) status of any contractor they engage directly. Get it wrong and HMRC can pursue the business, not the contractor, for unpaid tax. We cover this in full in a separate article.
• Best for: A narrow, well-defined piece of work with a fixed skill requirement and a business with the internal capability to manage IR35 status correctly.

Agency / software house

• Cost: Typically a day rate or monthly retainer covering a small team, usually £4,000-£15,000+/month depending on team size and seniority mix — higher than a single contractor day rate, but covering multiple disciplines (engineering, QA, sometimes design) without separate hires.
• Speed to start: Fast — an established agency can usually start within 1-3 weeks, with a team that already works together.
• Continuity: Strongest of the three — the contractual relationship is with the business, not an individual, so illness, holiday or turnover on the agency side is the agency’s problem to cover, not yours.
• IR35 exposure: None for a genuine B2B services contract — the agency employs its own engineers and manages their status itself.
• Best for: Time-bound projects, work needing multiple disciplines, and any business that wants delivery risk to sit with the vendor rather than internal management.

The cost comparison most people get wrong

A £600/day contractor sounds cheaper than a £700/day agency engineer until you account for what is actually included. A contractor’s day rate covers exactly one person’s time. An agency day rate typically also covers project management, code review from other senior engineers, QA, and cover during holiday or illness — work that, done in-house, would otherwise need separate budget or simply would not happen. The honest comparison is total delivered output per pound, not day rate alone.

A simple decision matrix

• Need it started this month, scope is still evolving, and risk should sit with the vendor: agency.
• Need one specific, well-defined skill for a fixed period, and you can manage IR35 status correctly: contractor.
• Building a genuinely long-term product with stable headcount and the management structure to support a team: in-house, often alongside an agency for surge capacity.

Hybrid approaches work well in practice

The three models are not mutually exclusive. A common, sensible pattern: a small in-house team owns the product long-term, backed by an agency retainer for surge capacity on major features or a specific technical migration. This gets you continuity where it matters (product ownership, roadmap knowledge) and flexibility where it does not (temporary spikes in build capacity) — without the overhead of hiring for headcount you only need for six months.

A 12-month total cost of ownership example

For a single senior engineer’s worth of capacity over a year, the three models land in a similar place more often than the headline rate suggests, once every cost is counted:

• In-house senior hire: £75,000 salary + ~25% employer costs + recruitment fee (often 15-20% of salary) ≈ £105,000-£115,000, plus 6-12 weeks of zero output while hiring, plus onboarding ramp-up time before they are fully productive.
• Contractor at £650/day, 46 working weeks: ≈ £150,000, with no employer costs but the IR35 administrative burden if engaged directly, and full delivery risk if they are unavailable or leave.
• Agency retainer covering equivalent senior capacity: typically £90,000-£130,000/year depending on scope, but including cover for holiday and illness, senior peer review, and no recruitment or IR35 administration cost.

The numbers converge more than most people expect. The real differentiator is usually not cost but risk allocation: who absorbs the impact if the work does not go smoothly. In-house, that is you. With a contractor, that is you. With an agency, it is contractually the agency’s problem to solve.

Red flags when evaluating any of the three

• In-house: a job spec so broad it is really three roles in one — a strong signal the hire will be stretched too thin to do any of them well.
• Contractor: reluctance to discuss IR35 status or insistence that “it does not apply to us” without a documented determination.
• Agency: account management that cannot answer specific technical questions, or an unwillingness to name the actual engineers who will do the work before you sign.
• Any of the three: no clear answer to “what happens if this person or team is unavailable for two weeks” — the answer reveals your real continuity risk.

Frequently asked questions

Can a contractor and an agency work on the same project?

Yes, though it works best when responsibilities are clearly divided — for example, a contractor owning a specific integration while an agency team owns the core build — rather than both groups working the same codebase without a single point of technical ownership.

Is it ever worth hiring in-house just for one project?

Rarely. The sunk cost of recruitment and the awkwardness of managing someone out once the project ends usually make a fixed-term agency engagement or a well-managed contractor arrangement more sensible for genuinely one-off work.

How do I evaluate whether an agency team is actually senior?

Ask who specifically will work on your project, not just who the agency has worked with before — and ask to speak to the engineers directly, not only account management. A team that avoids that conversation is usually telling you something.

Questions worth asking before you commit

• Is this a one-off build or an ongoing product? That answer alone rules out at least one of the three models.
• Who bears the risk if the person doing the work is unavailable for two weeks?
• Do you have the internal capacity to manage a contractor’s IR35 status correctly, including documentation?
• If a hire or contractor leaves mid-project, who has the knowledge to keep it moving?
• Does the work need more than one discipline (engineering, QA, design, DevOps) — and if so, who is coordinating that?

We work as the agency option in this comparison — a senior, employed team, engaged as a retainer or fixed-scope project, with no IR35 exposure and no single point of failure. If you are weighing this decision for a specific build, talk it through with us — we will tell you honestly if an agency is not actually the right fit for what you are building.