How to Read a Building Quote: PC Sums, Provisional Sums and the Exclusions Page
By Anderson & the LCCL Construction Team | Finchley N3
Clients send us other companies' quotations every week and ask whether the number is fair. The honest answer is usually that the number is fine; it is the words around the number that decide what you will pay. A quotation for a single-storey rear extension can read £108,000 on the front page and come to £125,000 by handover without anyone doing anything wrong, because the difference was written into it from the start as PC sums, provisional sums and exclusions.
This guide explains those three terms, walks through an anonymised quote a client shared with us recently for a rear extension on a terraced house in London, and gives you a way to put two proposals side by side on the same scope. No company is named and the figures are rounded.
The three terms that move the price
PC sum (prime cost sum). An allowance for an item whose specification is not yet decided — kitchen units, sanitaryware, tiles, bi-fold doors. The builder includes, say, £5,500 for the doors; if the doors you choose cost £8,000, you pay the difference. A PC sum is not a quote for the item, it is a placeholder for it.
Provisional sum. An allowance for work whose extent is not yet known — glazing before the structural opening has been surveyed, drainage before a CCTV survey, underpinning before a trial pit. It is re-measured and re-priced once the facts are known. Provisional sums nearly always go up, because they are set at a "basic" level by definition.
Exclusions. The list of things the price does not include. On a management-style quotation it is often a separate page: bathrooms, chimney breasts, patios, fire doors, interlinked alarms, drainage surveys, the building inspector's own fee, party wall surveyors, parking permits, bay suspensions, worktops, flooring, appliances. Each one is a variation when it arrives.
A worked example: the quote on the table
The quotation we were sent was well laid out — four sections and a total — which is exactly why it is useful. In round figures:
Shell and core, about £55,000: demolition and groundworks, steel, the extension structure, the roof and first-fix services. Firm.
Additional items, about £22,000: demolishing a conservatory, a new manhole, opening up the porch, forming a utility room, underfloor heating, a new boiler, and taking out a front window to get the steel in. Firm, but each one a line you could miss on first reading.
Glazing, about £12,500, marked PC sum and provisional: bi-folds, three skylights and a side window, to be surveyed and re-priced after the opening is formed, with a note that the nominated supplier may not be able to make the design.
Finishing, about £18,000: decoration and flooring in the kitchen, utility and hall (client supplies the flooring and adhesive), installing a kitchen the client supplies, with worktops by a separate specialist.
Total on the front page: about £108,000 + VAT. Then the client-information pages: a fee per structural engineer's review, a fee for an electrical layout, a fee for the water company build-over application, a weekly charge for a site toilet, a 24-month defects period that sits with a separate contractor, and an exclusions list that includes any fire doors or alarms the inspector asks for on an open-plan ground floor — which, on an open-plan ground floor, the inspector will. Put realistic numbers on the glazing, the kitchen worktops, the flooring and the fire precautions and the same job is £120,000 – £125,000 + VAT before anything goes wrong.
None of that is sharp practice. It is how that model prices. The point is that the £108,000 is not comparable with a fixed-price design & build figure for the same scope until you have added the allowances back in. Our four worked examples show what fixed prices look like for real projects, and the rear extension cost guide explains the ranges.
How to compare two quotes on the same scope
Write the scope once. Floor area, finish level, kitchen, bathroom, glazing spec, structural openings into the existing house, boiler, UFH. Then price each quote against that list, not against its own front page.
Move every PC and provisional sum to a realistic figure. Ask each company what the same items usually cost on their jobs. If they will not say, use the upper end.
Add the exclusions you know you will need. Fire doors and interlinked alarms on an open-plan ground floor, the building inspector's fee, a party wall surveyor if either neighbour is likely to dissent.
Add the per-item fees. Structural reviews, drawings, applications, site welfare. Small individually, several thousand together.
Then compare the warranties. 12 months on workmanship and 10 years insured on structure under one contract, or 24 months from a contractor and 3 years from a manager under two. See who actually builds your extension for why that matters.
Anderson's advice
“Clients budget for kitchen units but forget structural steelwork changes when opening up ground floor walls.” The quote above had the steel for the extension in the shell price — and a separate line for taking out a front window to get the beam into the house, which tells you the structural work had been thought through. What it did not have was a firm price for the opening between the old kitchen and the new room, because that depends on the engineer's review, which was a fee per review. When you read a quote, find the steel: where it is, what it includes, and whether the engineer who designed it is paid by the company that is installing it.
What a fixed-price quote looks like
As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — our quotations are a fixed price for the scope on the drawings, with the structural design, the building control application, the glazing and the finishing level priced in, and the only client-supplied items being the ones you choose to supply. The instant estimate gives you the range for your own house in two minutes, itemised line by line; a 30-minute call turns it into a scope we can both price against.
What our clients say
“LCCL Construction were professional from start to finish. The team were trustworthy, reliable and delivered work to the highest standard.”
— KashImaging, Trustpilot
Frequently asked questions
What is a PC sum in a building quote?
A prime cost sum: an allowance for an item not yet specified — doors, kitchen units, sanitaryware. If your choice costs more than the allowance, you pay the difference. It is a placeholder, not a price.
What is the difference between a PC sum and a provisional sum?
A PC sum covers an item whose specification is undecided; a provisional sum covers work whose extent is unknown until something is opened up or surveyed — glazing after the structural opening, drainage after a CCTV survey. Both are re-priced later and both usually rise.
How much do exclusions add to a building quote?
On the anonymised quote in this guide, realistic figures for the glazing, worktops, flooring and fire precautions take a £108,000 headline to roughly £120,000 – £125,000 + VAT. Every quote is different, but 10–20% above the headline is common on management-style schedules of works.
Get a figure for your own project
To book a free 30-minute consultation call about your extension or renovation, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.
One team, one contract, one point of contact — from first sketch to handover.
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