Why is one builder's quote so much cheaper than another?

Usually because it contains less, not because the cheaper builder is more efficient. The three places the difference hides are the fixings and fittings nobody names, the materials described by category rather than by product, and the trades that were never listed at all — fire-stopping, cavity trays, ventilation, making good. Labour rates between London contractors vary far less than most people expect. Scope varies enormously.

How do I compare two builder quotes properly?

Make them describe the same thing before you compare the numbers. Send both contractors the same written scope with named products where you care about the product, and ask each to price exclusions explicitly. If one quote says 'rooflight' and the other says a manufacturer and a model with a stated U-value, you are not looking at two prices for one job — you are looking at two different jobs that happen to be in the same house.

What is the difference between prescriptive and descriptive specification?

Prescriptive information tells the builder exactly what to install — this product, this thickness, this fixing. Descriptive information sets a performance requirement and leaves the solution to the contractor or a specialist. Both are legitimate, but only prescriptive information makes two tenders genuinely comparable, because both contractors are pricing the same object. A quote built on descriptive wording can be honest and still land somewhere you did not expect.

Do the small fixings really change the price that much?

Individually no, cumulatively yes, and in reliability terms they matter far more than their cost. Joist hangers fixed with the manufacturer's specified square twist nails rather than whatever was in the pouch, wall ties in the right material grade with at least 50mm embedment in both leaves, correctly sized lintel bearings, proper fire-stopping instead of expanding foam — none of these are expensive. All of them are invisible once the walls close, which is exactly why they are the first things to disappear from a keen price.

Should I always choose the more expensive quote?

No. The point is not that dearer is better — it is that you cannot tell which is better until both quotes describe the same work. Sometimes the cheaper contractor has simply been more efficient about preliminaries, or has a trade in-house that the other subcontracts. Once the scopes genuinely match, a lower price is a real advantage rather than a warning. Compare like with like first, then take the better number with confidence.

Renovation

Two Quotes, One Job: Stop Asking Which Is Cheaper and Ask What Is Inside the Price

The gap between two quotes for the same job is usually 3 things you cannot see: the fixings, the named materials, and the trades nobody listed.

By Vladimir Castravet·17 August 2026

Two quotes for the same job usually differ in 3 places, and none of them are visible when the work is finished: the fixings nobody names, the materials described by category instead of by product, and the trades that were never listed at all. Wall ties at 2.5 per m², lintel bearings of 150mm, weep holes every 900mm — none of these are expensive, and all of them vanish behind plaster. Labour rates between London contractors vary far less than people expect; scope is what varies. The estimator gives you a line-by-line breakdown to hold both quotes against, and our extensions and loft conversion pages set out what a full scope actually contains.

A developer once put two quotes for the same job in front of me and asked which one was right. So I explained it with a roof — because the difference between those two prices was entirely inside the parts nobody would ever see.

Two Prices, One Job: Why The Cheap Quote Costs You More

Two roofs, from the top

Imagine two rear extensions, same size, same rooflight, same finish. From the garden in five years they look identical. Inside the build-up, they are not the same object.

On the first, the joist hangers are fixed with the manufacturer’s specified square twist nails, into every hole. On the second, they are fixed with round wire nails from whatever box was open, into about half the holes. A hanger is only rated when it is fixed as tested. Nobody will ever see this.

On the first, the cold roof is ventilated: a continuous 10mm gap at the eaves for a pitch over 15°, 25mm where the pitch is 15° or less, and 5mm at the ridge where the pitch exceeds 35° or the span exceeds 10m. On the second, the insulation was pushed tight to the eaves and the path is blocked. For a year nothing happens. Then the roof void starts running with condensation in February, and the first symptom is a stain on a bedroom ceiling that everybody blames on the tiles.

On the first, every service penetration through a fire-separating element is sealed with a tested fire-stopping system. On the second, the gaps are filled with expanding foam — which is not fire-stopping, has never been fire-stopping, and is the single most common thing I find behind plasterboard.

Neither of those roofs is visibly wrong on handover day. One of them is still quiet in year five.

The figures that get quietly dropped

| The detail | What the standard actually requires | Source | |---|---|---| | Wall ties | 900mm horizontal × 450mm vertical, or not less than 2.5 ties/m². Extra ties not more than 300mm apart vertically within 225mm of the edges of all openings. | AD A §2C8 | | Tie specification | To BS EN 845-1, material reference 1 or 3 (austenitic stainless steel), embedment not less than 50mm in both leaves. | AD A §2C19, Table 5 | | Lintel bearing | 150mm or greater each end; may reduce to 100mm where the clear span is 1200mm or less. | AD A §2C24(a) | | Cavity tray and weeps | Weep holes every 900mm. Where the tray stops — above every window and door — stop ends and at least two weep holes. | AD C §5.5(c) | | Fire-stopping | Every joint, imperfect fit and service opening through a fire-separating element sealed so the fire resistance of the element is not impaired. | AD B Vol 1 §9.1 | | What is not fire-stopping | Materials must be reinforced with or supported by materials of the appropriate class. Expanding foam alone is not fire-stopping. | AD B Vol 1 §9.25–9.27 | | Cold roof ventilation | 10mm continuous at eaves over 15° pitch, 25mm at 15° or less, plus 5mm at the ridge over 35° pitch or 10m span. | AD C §6.11 / BS 5250 | | Extract ventilation | Kitchen 30 l/s with a hood to outside, 60 l/s without. Bathroom 15 l/s. Utility 30 l/s. WC 6 l/s. | AD F Vol 1 Table 1.1 |

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; your building control body has the final word on your specific building.

Add up what it costs to do every one of those correctly on a single-storey rear extension and it is a small number. Leave them all out and the quote drops by a noticeable amount, and nothing looks different for two years.

The three places the money hides

1. The fixings and the fittings. Hangers, ties, straps, restraint, padstones, cavity closers, trays, weeps, fire collars. Each is cheap. Together they are a real number, they are all invisible after first fix, and they are the structural and moisture integrity of the building.

2. Materials named by category, not by product. “Rooflight” is a category. A manufacturer, a model and a stated U-value is a product. The gap between the cheapest thing that answers to “rooflight” and the thing you had in mind is often thousands, and the quote is not technically wrong — you simply never said which one.

3. The trades nobody listed. Fire-stopping. Cavity trays. Ventilation and commissioning. Making good after the electrician. Scaffold licence. Skip permits. Drain CCTV before you build over a run. Testing. These are not extras. They are the job. But if a quote does not name them, they are not in it, and the conversation about them happens later, from a position where you have already committed.

Make them describe the same thing first

This is the whole method, and it is simpler than it sounds.

Write one scope and send it to both. It does not have to be a professional specification. It has to be the same words going to both contractors. The moment each builder is pricing their own idea of the job, the two numbers stop being comparable and no amount of staring at them will fix it.

Name products where you care about the product. Rooflights, windows, doors, kitchen, sanitaryware, flooring, ironmongery. Everywhere else, say what performance you need and let them solve it — but know which you have done. In design terms, prescriptive information tells the builder exactly what to install; descriptive information sets a requirement and leaves the answer open. Only prescriptive information makes two tenders genuinely comparable, because only then are both contractors pricing the same object.

Ask for exclusions in writing. Not “is anything excluded?” but “list what is excluded”. A contractor who has priced carefully has this list already and is usually relieved to be asked. A contractor who has not will produce it during the build, one item at a time.

Ask what each assumes about the unknowns. Foundation depth is decided by the ground and confirmed by building control in the open trench, not on the drawing — so ask what depth each quote assumed and what happens if the trench says otherwise. Same with the condition of existing drains, the state of the existing roof structure, and whether there is asbestos. A quote that states its assumptions is not hedging; it is telling you where the risk sits.

Then compare. Once both are describing the same building, a lower price is genuinely a lower price, and you should take it with confidence.

What a good answer sounds like

You are not testing whether a builder is expensive. You are testing whether they have thought about your specific house. Ask what is inside the price and listen for specificity:

“We’ve allowed 1.0m foundations because of the plane tree three doors down, and if the trench comes up shallower you get the difference back.”
“That rooflight is the model you sent me. If you want the frameless one it’s a different number and I’ll price it separately.”
“Fire-stopping and cavity trays are in the price. I’ll photograph both before the boards go on.”

Compare that with “it’s all included, don’t worry”. The second is not necessarily dishonest. It is simply not information, and you cannot buy on it.

The companion question is the payment schedule — where the money sits in time rather than what it buys. Our lesson on front-loaded pricing covers that half.

The takeaway

Do not ask which price is cheaper. Ask what is inside the price — the fixings, the named materials, the invisible trades, the assumptions. Both roofs look the same from the top. Only one of them is still quiet in year five.

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