Provisional sums and PC sums: why a cheap quote can grow

Two quotes arrive for the same extension, and one is noticeably cheaper. Before celebrating, look for the lines marked PS or PC. That is often where the difference is hiding.

Both are allowances: amounts included in the price for something that hasn't been fully designed, chosen or uncovered yet. They are perfectly legitimate. Every builder uses them, and on some items there is no honest alternative. The trouble is that they are set by whoever writes the quote, and the lower they are set, the cheaper the quote looks.

Provisional sums

A provisional sum is an allowance for work that can't be properly defined when the job is priced.

Foundations are the classic example. A builder can price foundations to a standard depth, but nobody knows what the ground is like until it is dug. If trees, clay or soft spots mean building control wants them deeper, the cost goes up, and no amount of careful estimating could have pinned that down in advance. A provisional sum for foundations is often the honest way to deal with it.

Other common ones are drainage alterations, repairs to existing structure that can only be assessed once it is opened up, and anything still waiting on a design, such as a feature staircase or a piece of bespoke joinery.

When the work is actually done, the provisional sum comes out and the real cost goes in. If the real cost is higher, you pay the difference.

PC sums

A PC sum, short for prime cost, is an allowance for buying a specific item you haven't chosen yet. Tiles at a rate per square metre, sanitaryware, a kitchen, light fittings, door handles.

Usually a PC sum covers supply only, and the fitting is priced elsewhere in the quote. If you choose something that costs more than the allowance, the difference is added. If you choose something cheaper, it comes off.

The trap is that an allowance can quietly decide your specification for you. If the tile allowance only buys a basic tile, the large-format porcelain you had in mind is an extra before anyone has picked up a trowel, and it often costs more to lay as well.

A provisional sum is a placeholder, not a price. The lower the placeholder, the cheaper the quote looks, and the bigger the bill when the real thing is chosen.

How a cheap quote grows

The pattern usually goes like this. The quote has a modest allowance for the kitchen, a low rate per square metre for tiles, and foundations priced to the shallowest depth anyone could hope for. It wins the job. Then the kitchen the client actually wanted costs more than the allowance, the tiles chosen are well above the rate, and the foundations go deeper.

None of that is really a variation. It was always going to happen. It just wasn't in the number.

A contingency is something different again. It is money you set aside yourself for the genuinely unforeseen, and it should sit on top of the quote, not be hidden inside it.

How to read a quote with allowances in it

• List them. Go through every quote and write down each provisional and PC sum with its amount.
• Total them. If a large share of the price is allowances, you don't really have a fixed price yet.
• Ask what each one buys. What kitchen does the kitchen allowance get you? Does it include worktops and appliances? What tile does the tile rate buy?
• Ask how they're adjusted. Will the builder add a margin to the actual cost? Is fitting included, or extra?
• Level them across quotes. If one builder allows much less for the kitchen than another, put the same figure into both before you compare. It's the only fair way to see which is cheaper.
• Decide earlier. Every item you choose before signing can be turned from an allowance into a fixed price.

I've written more about laying quotes side by side in how to compare builders' quotes like for like, and the wider questions to ask are in eight questions to ask a builder before you sign.

Getting to a number you can hold

The best way to keep provisional sums down is to resolve the design before it is priced: fixed drawings, a structural design, and the main finishes chosen. What is left as a provisional sum should be the things that genuinely can't be known yet, like the ground, and each one should say what it covers and how it will be adjusted.

Our working rates are on the cost guide. Send us your drawings and specification, and we'll tell you which parts of the price can be fixed now, which can't yet, and why.