A set of architect's drawings tells you where the walls, doors and windows go. It rarely tells you what grade of insulation sits in the roof, which brand of boiler is being fitted, or whether the plastering includes making good around a new opening. That gap between drawing and specification is exactly where budgets go wrong, because two builders pricing the same drawing without a shared specification are not pricing the same job.
Producing the specification is a distinct piece of work, usually done alongside or just after the drawings, and it's worth understanding how it gets built up.
Listing every element by trade
A specification works through the job trade by trade — groundworks, structure, roofing, external envelope, first fix services, plastering, joinery, decoration — and states what happens at each stage rather than describing the finished result in general terms. This structure is what allows a builder to price accurately, because vague scope forces guesswork into every line.
On a kitchen extension, for example, the specification separates the structural opening, the new roof build-up, the window and door supply, the first fix electrics and plumbing, the plastering, and the finishes, rather than lumping them together as 'kitchen extension' and leaving each contractor to interpret the gaps differently.
Stating materials, grades and finishes by name
Every material that affects cost or appearance should be named, not described generically. 'Insulation to current Building Regulations' is a starting point but 'insulation' alone tells nobody anything, since thickness, type and where it sits in the build-up all change both performance and price substantially.
The same applies to sanitaryware, ironmongery, worktops, flooring and roof coverings — naming a specific product or an equivalent standard prevents the builder from substituting a cheaper item without your knowledge, and prevents a dispute later about what 'standard fittings' was ever supposed to mean.
Defining exclusions as clearly as inclusions
What's left out of a specification matters as much as what's in it. Decoration, floor coverings, external landscaping and appliance supply are commonly excluded from a building contract, but only if that's stated plainly rather than assumed by one side and not the other.
A specification that lists exclusions explicitly, alongside inclusions, gives you a document you can hold a contractor to later, rather than one open to reinterpretation once the invoice arrives.
Setting out who supplies what
Client-supplied items — a kitchen bought directly, tiles chosen from a specific supplier, bathroom fittings sourced separately — need to be identified in the specification along with their expected delivery date, because the programme depends on them arriving on time and undamaged.
Where the builder is supplying an item, the specification should state the make or an agreed equivalent, so there's no ambiguity about whether a substitution meets the intended standard.
Why this document is what makes quotes comparable
Two quotes for the 'same' extension can differ by tens of thousands of pounds not because one builder is dishonest, but because they were pricing different unwritten assumptions. A shared specification removes that variable, so any difference in price reflects a genuine difference in labour rate, programme or margin rather than a difference in what's actually being built.
This is why a specification is worth commissioning even on smaller jobs where a full architectural package feels unnecessary — a page of clearly stated materials and inclusions still does the job of making quotes genuinely comparable.
Common questions
Who writes the specification — the architect or the builder?+
Either can, and sometimes both contribute. An architect or designer often produces the initial specification alongside the drawings, and a builder may then add trade-specific detail once appointed. What matters is that one document exists and both sides are working from it.
Is a specification necessary on a small job?+
It's proportionate to the job, but even a modest bathroom refit benefits from a short written list of materials and inclusions, since verbal agreements are the most common source of dispute on small works.
What does 'provisional sum' mean in a specification?+
It's a placeholder figure for work that can't be priced accurately until it's uncovered or a decision is made — for example, unknown ground conditions or a tile allowance before you've chosen tiles. A specification should flag every provisional sum clearly, since these are the figures most likely to change.
Get a written specification for your project
We'll produce a trade-by-trade specification you can use to get genuinely comparable quotes, or price directly ourselves.