What drawings do builders need to price an extension in London?

The drawings a builder needs for a like-for-like quote

A quotation set is the building regulations package with a specification attached. Each sheet answers a pricing question, so nothing is left for the builder to assume.

The law already expects most of this to exist. For a full plans application, the Building Regulations 2010 say plans are only full plans if they include any other plans which are necessary to show that the work would comply with the regulations. Drawing it once, before quotes, means the builder prices the same set building control approves.

Comparing quotes like-for-like

Three quotes for the same extension can differ by far more than any builder's margin. Usually the gap is not the labour rate; it is what each builder decided the job was. With a full drawing set and specification, every builder is pricing the same foundations, the same steel and the same roof, so the difference that is left is genuinely about price, programme and how they work.

Read each quote against the drawing list. Look for items priced as provisional sums or prime cost sums, which are allowances rather than fixed figures. A provisional sum for the steel, the foundations or the drainage usually means the drawings did not answer that question. Ask each builder to price the same items as fixed figures from the drawings, and the quotes line up.

If you plan to open the back wall of the house, the beam is often the largest single unknown. What drawings you need to remove a load-bearing wall explains who designs it and what the builder receives.

What the specification adds to the drawings

Drawings show where things go; the specification says what they are. It names the insulation product and thickness, the block type, the roof membrane, the window performance and the standards the work must meet. Without it, two builders can both build the drawn extension and price materials of very different quality.

On our sets the key specification notes sit on the drawings themselves, next to the element they describe, so the builder pricing the job and the one building it read the same thing. Where you want a particular finish, such as a specific rooflight or flooring, we note it so it is priced rather than added later.

Who is responsible for giving the builder this information

The Construction (Design and Management) Regulations 2015 say a client must provide pre-construction information to every designer and contractor appointed, or being considered for appointment. That includes builders who are only quoting.

For a domestic client, regulation 7 passes those client duties to the contractor for a project where there is only one contractor, or to the principal contractor where there is more than one. In plain terms, your builder carries the legal duty, but they still need accurate drawings to plan the work safely and price it. A clear set helps them meet that duty instead of working it out on site.

Full plans or a building notice: what it means for quotes

With a full plans application, building control checks the drawings before work starts. GOV.UK says you can expect a decision within 5 weeks, or 2 months with your consent. Builders can then price an approved set, and the approved set is what the inspector compares the work to.

A building notice lets work start 2 days after your notice has been submitted, but GOV.UK is clear that you do not get formal approval like you do with full plans. That suits small jobs; on an extension with steel and new drainage it leaves more decisions to site, and more room for extras. Our guide to what building control needs for an extension goes through the officer's checks sheet by sheet.

Drainage deserves its own line in any quote. Under the regulations a full plans set must give particulars of the precautions to be taken in building over a drain, sewer or disposal main where that rule applies, and many London terraces have a shared sewer under the back gardens.

What our building regulations drawings include

Our building regulations drawings cost £745 as a fixed fee, with the measured survey included. You get the plans, sections, construction details, drainage layout and specification notes in one set that building control can approve and builders can quote from. Structural calculations come from our structural drawings service at £745, and the member sizes go onto our sheets so the two match.

If you have not had planning drawings yet, our house extension drawings cover that first stage. The cost of each route is set out in how much building regulations drawings cost.