1. Structural work & building envelope
Anything that affects the “bones” of the house (roof, walls, foundations, major extensions) tends to drive big costs. For example:
- According to HomeOwners Alliance, putting in an extension (in many cases) costs tens of thousands of pounds. HomeOwners Alliance+2Bristol Builders+2
- A recent guide shows structural/major building work dominating cost examples: one full-renovation breakdown listed “structural and building work” as the biggest line item. Style Within
- Also, issues like roofs needing replacement: “one of the biggest house renovation costs can arise if you need to replace the roof … £4,500-£12,000” in many cases. HomeOwners Alliance
Why is it so expensive? Because structural work is complex: it involves skilled labour (brickwork, roofers, engineers), materials (roof tiles, structural steel, blocks), possibly scaffolding, planning/permissions, and tends to disrupt everything else if done mid-project.
2. Kitchens & bathrooms
These are rooms that combine a lot of different trades (plumbing, electrical, tiling, cabinetry), and because finishes and fixtures vary wildly, costs can escalate.
- One report states: “Kitchens are the most expensive rooms to renovate” in the UK. Property Rescue
- Another source shows a kitchen renovation (including appliances, high spec) costing in the order of £12,500-£30,000 for a 2-bed house. hillarys.co.uk+1
Why are they costly? Because you’re not only updating surfaces, but also services (plumbing, wires), appliances (fridge, oven, extractor), finishes (countertops, cabinetry), and often the layout (knocking walls). These combine into a major budget chunk.
3. Services: plumbing, electrics, heating & insulation
Updating or replacing the hidden systems of the house—electric wiring, plumbing pipes, heating systems, insulation—can be very expensive, especially when the property is older or has been neglected.
- For example: rewiring a home can run into the thousands. Homebuilding+1
- Upgrading heating, insulation and major service work is flagged as a cost driver. Bristol Builders
Why expensive? Because service upgrades often require pulling out walls/floors, compliance with building regs, skilled trades, and new materials that aren’t “pretty finishes” but essential for safe, legal, and efficient operation of the home.
🎯 Which of these is the most expensive?
If we pick a single item that typically has the highest cost and greatest variation, it would be structural work / extensions / major building-fabric upgrades.
- An extension adds new floor area, walls, roof, windows, often ties into the existing building fabric and services — so it multiplies many cost drivers (materials, trades, design, disruption).
- Among the structural/major works, “roof replacement” is flagged explicitly as “one of the biggest costs” by the HomeOwners Alliance. HomeOwners Alliance
- Also, the structural breakdown from Scenario in turn0search11 shows structural/building work dominating major renovation budgets.
So while kitchens and services cost a lot, if you add new space or repair/replace major fabric (roof, walls, foundations), that often sits atop the cost pyramid.
✅ Takeaways & what to watch for
- Budget early for structural/service works before finishes. The finishes (paint, flooring, cabinetry) are visible and satisfying, but if you skip or underestimate the building fabric or services, you’ll likely blow the budget.
- Get a survey (especially for older homes) to check roof, walls, services — so you know if there are hidden major costs.
- Contingency is key. Unexpected issues (damp, rot, old wiring, structural weaknesses) often surface during renovation. Many sources recommend at least 10-20% contingency. HomeOwners Alliance+1
- Cost vs value: Doing a huge extension or ultra-luxury kitchen may cost a lot but doesn’t always guarantee equal value uplift, especially if the local market doesn’t support that level.
- Phasing is an option: If budget is tight, you might prioritise structural and service improvements, then phase the high-end finishes later.
