Whatever the project (an extension, a loft conversion, a full house build) pricing extensions and house builds properly comes down to the same job: measure it off the drawings, price it on real rates, and deliver it before the deadline. The size changes. The method shouldn't.
That sounds obvious. In practice, plenty of builders use three different methods depending on the job. The house build gets a proper breakdown because the numbers are big enough to be frightening. The extension gets priced from experience. The loft gets a figure off the top of the head during the site visit, and then gets written up to match.
Scale changes the size, not the method
A loft conversion has fewer elements than a full house build, but each one still needs measuring rather than estimating by eye. Skipping the method because the job's smaller is exactly how small jobs lose money on site.
Think about what actually differs between the three. A house build has substructure, superstructure, roof, envelope, internal walls, finishes, services and external works. An extension has almost all of the same headings, just with smaller quantities and the added complication of tying into an existing building. A loft has fewer headings but more awkward ones: working at height, structural alterations to an existing roof, a new stair that has to fit somewhere, fire protection to the escape route. None of them is simple. They're just complicated in different places.
So the discipline is the same for all three. Work through the drawings element by element, measure each one to NRM2, and price what's actually there. The house build takes longer to measure. It doesn't need a different kind of thinking.
Why pricing extensions and house builds by the square metre falls short
Two extensions the same size can price very differently depending on finish and services. Measuring against the actual drawings and specification catches that difference; pricing off a rate per square metre doesn't.
Say you have two single-storey rear extensions with the same floor area. The first has a pitched tiled roof, standard windows, a laminate floor and two double sockets. The second has a flat roof with a large lantern, full-width sliding doors on a steel, underfloor heating, a polished concrete finish and a kitchen island needing a new waste run under the slab. Same footprint. Very different jobs.
A square metre rate is useful for one thing: a sense check at the very start, before there are drawings worth measuring. Once the drawings exist, it's the wrong tool. It averages away exactly the detail that decides whether you make money.
A rate per square metre prices the average job. Your client hasn't asked for the average job, they've asked for the one on the drawings.
The same steps, whatever you're building
Here's the sequence that works whether the job is a loft or a new-build house:
- Check the drawing set. Note the revision of every sheet and what's missing, such as the structural design or a services layout, before you measure anything.
- Measure every element. Take off quantities to NRM2, element by element, so each line in the estimate traces back to a drawing.
- Read the specification against the quantities. The same wall can cost very different amounts depending on the block, the insulation and the finish specified.
- Price on current, local rates. Labour and materials for the area the job is actually in, at today's prices.
- Write down the assumptions. Anything you've had to assume goes in writing, so the client knows where the price could move.
- Add your margin last. Once the cost is right, decide what you want to earn on it. Never the other way round.
Follow that on a loft and on a house build and you'll get two estimates that look alike. Different lengths, same shape. That consistency is useful in its own right: you can compare jobs, spot when a rate looks off, and hand any of them to a client without rewriting the layout.
Where each type of job tends to catch builders out
Each project type has its own weak spots. On a loft, it's usually the structural work and the stair, plus the making good below once the ceilings have been opened up. Even a modest loft conversion can hide steel, fire doors and a surprising amount of plastering.
On an extension, the weak spots sit below ground and at the junction with the existing house: foundations, drainage, and the work to join new to old. A proper quantity takeoff for an extension treats those as measured items, not an allowance.
On a house build, the risk shifts to the sheer number of items and the external works. Drives, paths, fencing, service connections and landscaping often sit at the back of the drawing set and at the back of the builder's mind.
One estimate you can bid from, whatever the project
Whether it's a small loft or a full new build, you should get the same kind of document back: measured quantities, priced on current rates, ready to bid from or hand to the client. That's how our construction estimating services work. Every element is measured off your drawings to NRM2, priced on current UK rates local to the job, for a fixed fee agreed before we start.
The result is an estimate you don't have to reinvent every time the project type changes. You check it, add your margin, and put your name to it.
That's the real benefit of pricing extensions and house builds to one method. Your clients get a document they can follow, and you get a price you can defend whichever kind of job it is.
A rule for any job on your desk
Before you price the next one, ask whether you'd use the same method if the job were ten times bigger. If the answer is no (if you'd measure a house build properly but you're about to guess the loft) then the method is the problem, not the size of the job. Measure it anyway.