Builders can give you a budget figure from planning drawings, but a firm price needs the building regulations drawings: the plans, sections and notes that show the structure, insulation, drainage and fire detail. For prices you can compare line by line, add a specification of finishes and fittings, or a full tender pack. Our building regulations drawings are £545 drawings + £150 survey (£695 total).
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Most London owners collect quotes too early, from the planning set, and are then surprised that three builders priced three different extensions.
What can a builder price from planning drawings?
The size and shape. Planning drawings must be to scale and show north (MHCLG, Making an application), so a builder can measure floor area, wall length and roof area. That supports a rate-per-square-metre budget, which is useful for deciding whether the project is affordable at all and for shortlisting builders.
What the planning set leaves open is most of the cost: foundation depth, the steel above the opening into the old house, how the floor and roof are insulated, where the drainage runs and whether a manhole must move. Each builder fills those gaps with their own assumptions. That is why planning-stage quotes on a typical London rear extension can differ widely, and why the smallest total is often the one that assumed least.
What does the building regulations set add?
The construction. The set we draw for building control shows wall, floor and roof build-ups, insulation to meet Part L (Approved Document L), the beam and padstone positions from the structural calculation, drainage runs, fire separation and ventilation. Our building control drawings post lists each item.
With that set a builder can price the shell firmly: groundworks, structure, envelope and services to first fix. You also need building control approval or a building notice before work starts in any case (regulation 12), so the set is not an extra; it is the set you need anyway, used one stage earlier.
London adds its own items a builder must know about to price: party wall work along the boundary, restricted access through a terraced house with no side gate, skip and parking permits on a controlled street, and sometimes a Thames Water build-over agreement where the extension sits over a shared sewer. Put those in a scope note so no builder silently excludes them.
Where do quotes still differ?
In the finishes and fittings. Neither the planning set nor the regs set says which windows, which flooring, how many sockets or which kitchen. One builder allows for a basic sliding door, another for a slim-frame one, and the totals stop meaning anything.
There are two fixes:
- Specify it. A schedule naming the window system, the floor finish, the sanitaryware and the electrical points, so every builder prices the same item.
- State a provisional sum. Where you have not decided, give every builder the same allowance, for example a stated figure for the kitchen, and tell them to include it unchanged. The quotes then differ only where the work differs.
A full tender package, from £945, does both and adds construction details at a scale a builder can work from. Our tender drawings cost post explains what is in it and when it is worth paying for.
What should I send each builder?
The same pack, every time:
- The approved planning drawings, or the submitted set if the decision is pending.
- The building regulations drawings and the structural calculation.
- A specification or schedule, with provisional sums where undecided.
- A one-page scope note: what is included (decoration, kitchen fitting, landscaping or not), London site items such as permits and access, and the date you want to start.
- The same questions: programme length, payment stages, what is excluded, and who handles building control inspections.
Ask each builder to price in the same order as your scope note. Then the comparison is a column check, not a guess. The HSE's guidance also matters: on a domestic project with one contractor, the client duties under CDM 2015 pass to that contractor, and with more than one, to the principal contractor (HSE). Confirm in writing who that is.
How do I compare the quotes when they come back?
Line them up against your scope note, item by item, before you look at the totals. Three checks catch most problems:
- Exclusions. A short list of exclusions on a low quote often hides the difference: no skip, no making good to the existing house, no decoration, no building control fee.
- Provisional sums. Check every builder used your allowance unchanged. A builder who cut the kitchen allowance has not quoted for the same job.
- Assumptions about the ground. Foundations are the commonest variation on London clay. A quote that assumes a standard depth with no allowance for going deeper, near trees for example, is not comparable with one that prices a stated extra per metre.
Where a quote differs on one item, ask that builder to re-price it to your scope rather than adjusting the figure yourself. Keep the answers in writing; they become part of the contract the builder signs.
Which set should I pay for before going to tender?
For a single-storey rear or side extension with a trusted builder, the building regulations set plus a short schedule is usually enough. For a larger or higher-spec job, or where you invite several builders, a tender pack earns its fee. Our planning and building regulations difference post explains the two main sets, and the building regulations drawings page has our scope and fixed fee.
Frequently asked questions
Can I get a fixed price from planning drawings alone? A builder may offer one, but it rests on their own assumptions about structure and finishes. Expect variations once the building regulations detail is drawn.
Should builders see the structural calculation? Yes. The steel size, padstones and any foundation changes are significant costs, and the calculation removes the guesswork.
How many builders should I ask? Enough to compare properly; three is a common habit, not a rule. More quotes do not help if each prices a different scope.
What is a provisional sum? A stated allowance for an item not yet chosen, which every builder includes unchanged. The final cost is adjusted when the actual item is chosen.
Can I send builders the drawings before planning is decided? Yes, for a budget figure and to book a slot. Tell them the scheme may change at decision, and ask for a firm price only once the approved and building regulations drawings exist, so nobody prices a design the council has altered.
