What should a building specification include for a home project?

A building specification describes the materials, performance, finishes and workmanship expected alongside the drawings. Drawings show where things go. The specification explains what those things are and the standard to which they should be supplied and installed. Without both, builders may price different work from the same plans.

What belongs in it?

For an extension or loft conversion, the useful detail usually covers:

• the drawing and revision register, plus any structural engineer’s design;

• foundations, structure, roof, insulation, windows, doors and fire measures;

• electrical, plumbing, heating and ventilation scope, including points and fittings;

• internal finishes, kitchens, bathrooms, tiling, decoration and external making good;

• product quality or performance requirements, acceptable alternatives and who selects them;

• testing, certificates, warranties, waste removal and protection of the occupied house.

Describe what is included, what the homeowner supplies and what is expressly excluded. Give quantities where available. A phrase such as “standard bathroom finish” is too elastic for reliable comparison.

Drawings and specification should agree

If the drawing shows three roof lights but a quote includes two, a dispute is waiting to happen. If structural details or service routes remain undecided, record the gap and its cost and programme consequence. Do not treat a planning layout as a complete build package; read our planning drawings guide.

The architect, engineer, designer and specialist contractors may each own parts of the technical information. A homeowner-side review can identify missing decisions; it does not replace their design responsibility.

How to use it when tendering

1. Issue every builder the same dated drawing and specification set.

2. Ask each to identify departures, alternatives and assumptions in a separate schedule.

3. Put homeowner-supplied items and provisional allowances on their own list.

4. Get written answers to queries and send the same answers to all tenderers.

5. Agree the final documents and revision numbers in the contract before work begins.

That creates a basis for comparing the actual scope, not just totals. Our guides on information needed for a quote, quote contents and allowances show how the documents connect.

Preparing to seek builder prices? Build Advocate can assess the available information, flag gaps and help you understand the budget and delivery risks. See our comprehensive £999 package.