Recruitment agency vs in-house hiring: which is right for your engineering roles?

Hire in-house when you have steady engineering volume, a strong employer brand and time to run a full search — at scale it's the cheapest route per hire. Use a specialist agency when a role is urgent, specialist or scarce, or when a wrong hire is costly. In the UK the average time to hire is about 4.9 weeks, and a single bad hire costs at least 30% of first-year salary — the number any agency fee is measured against.

The two routes, side by side

In-house recruitment looks cheaper because the cost is spread across salaries you already pay. But the true figure is rarely just the job-ad spend.

Benchmarks vary with what you count. Narrow, direct-cost surveys put UK cost per hire near £4,700; fully-loaded figures that include internal recruiter and manager time land around £6,125, rising to roughly £19,000 for a management-level role. [Talent Insight Group / CIPD, 2026] On top of that sit the hours nobody invoices for: an in-house recruiter may spend around 30 hours managing a single hire, and most roles pull several managers into screening and interviewing. [NatWest Mentor, 2026]

For comparison, an agency fee is a fixed percentage you can price up front: ON3 charges 7% (Agentic) or 20% (Atelier) with no deposit — £5,250 on a £75,000 engineer. Full detail is on the pricing page, and the market rates those percentages are measured against are in our how ON3's 7% and 20% compare to market rates.

For engineering specifically, the constraint usually isn't money — it's time and seniority. The people best placed to judge whether a candidate can actually build, scale and debug production systems are your senior engineers, and they're also the people you can least afford to pull off delivery for a week of interviews. That trade-off is the real cost of in-house technical hiring, and it's the gap a specialist agency is meant to fill.

In-house is the right answer when you have steady, predictable volume, a strong enough employer brand to attract inbound applicants, and the internal bandwidth to run a full, structured process. At scale, building that muscle in-house is cheaper per hire than paying a percentage fee on every role.

The headline benchmark for the UK hiring market is about 4.9 weeks from application to signed offer, consistent across several 2026 surveys. [StandOut CV; NatWest Mentor] Specialist and senior roles run longer — senior leadership hiring averages around 6.5 weeks, and vacant tech roles commonly take 6–8 weeks to fill. [StandOut CV; hireful] In London, where salary bands and competition are highest, an extra week of vacancy is the most expensive of all.

Every extra week a role stays open carries a cost that doesn't appear on any invoice: delayed delivery, overtime to cover the gap, and the risk of losing strong candidates — around 62% of candidates lose interest if a process drags. [NatWest Mentor, 2026]

This is where an agency's speed earns its fee. ON3 Works produces an AI-screened qualified shortlist in 24–48 hours and reports a median time to hire of under 30 days from signed brief to accepted offer. It's worth being precise about what 24–48 hours means: it's an AI-screened, qualified shortlist — not yet interviewed by a human engineer. In the Atelier tier, the senior-engineer technical interview follows before a candidate reaches the client. The value is in the combination — fast shortlist, then deep human vetting — not in pretending the two happen at once.

For most engineering roles, the deciding factor isn't the fee or the speed — it's the asymmetric cost of a wrong hire.

The US Department of Labor puts the cost of a bad hire at a minimum of 30% of first-year salary. [US DoL] SHRM's framework is wider, estimating replacement at roughly 0.5 to 2× annual salary depending on seniority. [SHRM, 2026] CareerBuilder data puts the average loss per bad hire at about $17,000 for entry- to mid-level roles, rising to around $240,000 for specialist or executive ones. In UK terms, a REC analysis found a manager-level bad hire on a £42k salary can cost around £132,000 once wasted salary, training and lost departmental productivity are combined.

Put concretely: at the conservative 30% floor, a wrong hire on a £120,000 senior engineer costs around £36,000 — before counting months of delayed delivery and a second search. Measured against that, an agency fee on a role you're at risk of getting wrong isn't the expensive option; it's the insurance.

In-house wins on cost at volume, on employer-brand building, and on roles where you have time and internal expertise to run the full process well. If you're hiring ten similar engineers a quarter and have a recruiting team and a senior bench to interview them, building that in-house is the economically rational choice.

A specialist agency wins on urgent, scarce or specialist roles, and on any role where a wrong hire is expensive enough that engineer-level validation pays for itself. ON3's own client pattern reflects this: most clients come to it from in-house or another agency, for exactly these harder roles. One example — a Madrid aerospace company needed five highly specialised cyber-threat-intelligence engineers for a new project, and ON3 delivered all five in under two weeks; six months on, one has already been promoted.

The honest version of this comparison is that it isn't either/or for most companies. Run high-volume, well-understood roles in-house. Reach for engineer-vetted agency support on the roles that are urgent, specialist, or too costly to get wrong. See also: how AI recruiting compares to a traditional agency.

Have a role that's too costly to get wrong?

Tell us who you need to hire. We'll come back with a qualified shortlist — and, for critical roles, a senior engineer who interviews every finalist.

• Pricing · Agentic vs Atelier — what each hire actually costs
• Guide · Recruitment agency fees UK — full 2026 breakdown
• Compare · AI recruiting vs a traditional agency
• Guide · Hiring engineers in the UK — salaries & market
• Guide · Hiring engineers in London

Sources & notes

UK time to hire (~4.9 weeks; senior ~6.5 weeks; tech 6–8 weeks): StandOut CV 2026 recruiter survey; NatWest Mentor, Time to hire in the UK (2026); hireful.

UK cost per hire (~£6,125; ~£19,000 management): CIPD / Talent Insight Group, via NatWest Mentor and Youth Employment UK (2026).

Bad-hire cost (≥30% of salary; 0.5–2×; $17k / $240k; £132,000 manager example): US Department of Labor; SHRM (2026); CareerBuilder; REC via StandOut CV. Candidate drop-off (~62%): NatWest Mentor (2026).

ON3 figures (7%/20%, no deposit; 24–48h shortlist; <30-day median hire; ~2.5% selectivity; 8–15+ yr engineer interviewers; 90.7% retention; 90-day replacement; CTI case study): supplied by ON3 Works.

Honesty caveat: the 24–48h shortlist is AI-screened-qualified, not yet engineer-interviewed; the engineer vet follows in the Atelier tier. Competitor and market figures are third-party 2026 estimates — re-verify before publishing.