We know. That number made you do a double-take.
And honestly? That’s exactly why we’re writing this post.
Because if you’re a homeowner planning a loft conversion or extension right now, you’ve probably been Googling costs and getting answers that range from “£30,000” to “who knows” — with very little in between that actually helps you understand what you’re going to spend.
So here’s what we’re going to do. We’re going to walk you through a real project — a genuine, recently-completed job we priced for a family home in Chelmsford — and show you every single pound, broken down by trade, by zone, and by scope.
No fluff. No vague estimates. Just the real numbers, and the story behind them.
Meet the Project: 21 Linge Avenue, Chelmsford
This one came to us through Revamp London, a high-end London contractor working on a family home in Broomfield, Chelmsford. The homeowner had a vision — not just an extension, not just a loft board-out — but a complete, architecturally ambitious transformation of their home.
Three separate scopes. All running at the same time. All done to an exceptional standard.
The Loft Conversion (38m²)
A full structural loft conversion with a brand new roof, two full-height dormers, and seven motorised skylights. Inside — two new rooms including a master bedroom with ensuite, a den area, and a beautifully engineered three-flight staircase with 28 oak-tread risers and a glass balustrade. The master bedroom floor? Microcement. Poured in-situ. The kind of finish you’d expect in a boutique hotel, not a suburban semi.
The First Floor Side Extension (15m²)
A new first-floor side extension to create a study/bedroom with ensuite. Timber and steel hybrid frame, cavity wall construction, fibre cement cladding on the outside. Fire-rated glazing where it meets the garage elevation. Small footprint — but serious specification.
The Ground Floor Glazed Canopy (~12m²)
A glazed canopy with a full brise soleil louvre system for solar shading, a 5.6-metre run of Schüco aluminium sliding doors, powdercoated metal cladding, and driveway works below. Architectural, considered, and — yes — expensive.
Total programme: 18 weeks. Total cost: £385,111.05 ex VAT.
The Full Cost Breakdown
Here’s the summary. Every number is from the actual QS report.
| Scope | Area | Cost (Ex VAT) |
|---|---|---|
| Preliminaries | — | £45,370.00 |
| Loft Conversion | 38m² | £190,256.00 |
| First Floor Side Extension | 15m² | £91,084.60 |
| Ground Floor Canopy | ~12m² | £58,400.45 |
| Total Ex VAT | 65m² | £385,111.05 |
| VAT (20%) | £77,022.21 | |
| Total Inc. VAT | £462,133.26 |
“Why Are Preliminaries £45,370?!” — We Hear You
This is the question we get most often. Preliminaries are the cost of running the site properly — the scaffolding, the site manager, the skips, the insurance, the welfare facilities, the cleaning, the health and safety. All the stuff that has to happen before and around the actual building work.
Most contractors bundle it into their overall quote so you never see it. A proper QS breaks it out so you know exactly what you’re paying for.
On this project it includes: site management for 18 weeks (£9,900); Contractor’s All Risk insurance (£1,800); Building Control fee (£900); Party Wall Act surveyor (£1,500); structural engineer inspections (£1,200); CDM co-ordinator (£800); Heras fencing, welfare facilities, H&S plan (legally required); full external scaffold — erect, hire, dismantle (£5,500); six skips plus a grab lorry for spoil; and final builder’s clean, defects liability period allowance, and O&M manual.
If a contractor quotes you for a job like this without visible preliminaries, ask where it is. Because if it isn’t priced at all, it’ll appear as a variation once work has started.
The Loft: £190,256 for 38m² — What Are You Actually Getting?
At £5,007 per square metre, this is not your average loft conversion. This is a new storey being added to the building — structurally engineered, architect-designed, and finished to a standard that adds serious value to the property.
The two full-height dormers are significant structural operations. Seven motorised skylights, microcement floors (a specialist trade — it can’t be done by a general labourer), and the three-flight oak staircase with glass balustrade is furniture-grade joinery. This is the loft conversion that genuinely transforms how a family lives in their home.
The Side Extension: £91,084 for Just 15m² — Why So Much?
First-floor extensions cost more because the structural engineering is significantly more complex. You can’t bolt a first-floor extension onto a house without dealing with what’s underneath it — structural calculations, potentially new foundations, integration with the party wall, and careful load transfer.
Add fibre cement and composite cladding, fire-rated glazing, and a full ensuite in a tight floor plan — and the cost stacks up quickly. Small footprint. High specification. High cost per metre. That’s the reality of first-floor work done properly.
The Canopy: £58,400 for 12m² — Seriously?
Yes. Schüco is a German aluminium systems manufacturer used on commercial and high-end residential buildings. A 5.6-metre run of Schüco sliding doors is a precision-engineered glazing system requiring specialist installation. The brise soleil louvre system is fully custom — designed, fabricated, and installed specifically for this building. Include external drainage, driveway works, and the full structural steelwork, and £58,400 is a reasonable number for what’s been delivered.
This isn’t a lean-to. It’s an architectural feature that makes the whole rear elevation sing.
Three Things Every Homeowner Should Take From This
1. The specification is the budget. The gap between a £40,000 loft conversion and a £190,000 one is almost entirely the specification choices. Oak vs. softwood. Microcement vs. carpet. Motorised skylights vs. fixed. Every upgrade compounds. Know your specification before you budget — not after.
2. If you can’t see the preliminaries, ask where they are. Transparent prelims mean a transparent contractor. If your quote is one total number with no breakdown, you have no way to challenge anything that comes later.
3. Cost per m² is a starting point, not a budget tool. £7,467.49/m² is the average across three very different scopes. The canopy is £4,867/m². The loft is £5,007/m². The extension is £6,072/m². Use scope-level figures — not averages — when planning your budget.
For Builders and Contractors Reading This
You already know that homeowners come to you with budgets that don’t match their brief. A QS report like this changes that conversation before it starts. When a client can see exactly where their money goes — prelims, trades, provisional sums — the conversation shifts from “why is it so expensive?” to “right, what can we adjust to hit our budget?”
That’s a better conversation for everyone.
Want a rapid turnaround detailed takeoff? info@rapidqs.co.uk
RapidQS is a UK quantity surveying practice specialising in high-spec residential extensions, loft conversions, and renovation projects. All figures in this post are from a live QS report, reflecting current UK contractor rates as of 2025.
