Senior Mechanical Design Engineer (Contractor-side) — Concept to RIBA Stage 4 London & South East | Commercial Fit Out | Main Contractor | Permanent A growing main contractor in the London and South East commercial office fit out market is strengthening its in-house design capability and is looking for a Senior Mechanical Design Engineer to lead from the front. Trading for 3.5 years and turning over circa £20m in 2026, the business has built its reputation quickly and now wants the right person to help shape how its mechanical design function develops. This is a contractor-side role with genuine ownership from feasibility and concept through to RIBA Stage 4 technical design. The focus is firmly on delivery: programme, procurement, installability, change control and margin. You will lead mechanical design across multiple projects, coordinate external consultants and design partners, and challenge outputs where needed, including reviewing IES, Revit and Hevacomp information. You will produce feasibility schemes and option studies to support pre-con decisions, and when budgets tighten you will lead value engineering conversations with confidence, presenting practical alternatives that protect performance and quality. You will also support tender and proposal submissions, attend design and client meetings, and produce clear actions, trackers and reporting, working closely with the commercial team on subcontractor tender analysis. They need someone with contractor-side mechanical design experience (D&B, design assist, or delivery-led design), who can evidence project ownership and interface management, and who is confident carrying designs through to Stage 4. You should be comfortable across Cat A, Cat B, turnkey and D&B environments, present well with clients, and able to explain design decisions in commercial terms, not just technical language. Strong Excel and PowerPoint skills are important, alongside a minimum HNC/HND in Building Services Engineering (or equivalent) and typically five or more years’ relevant experience with D&B exposure. Key points * Contractor-side mechanical design lead, concept to RIBA Stage 4 * Delivery-led decision making: programme, procurement, installability, change control, margin * Commercial fit out focus across London and the South East * Mechanical packages typically £250k–£1.5m, overall project values up to circa £4m * £80,000–£90,000 + package, travel covered/allowance, bonus, 25 days holiday + bank holidays + birthday This won’t suit a consultancy-only profile with no contractor-side delivery exposure, or anyone unable to demonstrate ownership beyond producing design outputs. If you want a role where design is judged by what gets built, and you have the scope to shape an in-house mechanical capability within a growing contractor, this is worth a conversation