Your Extension Specification Schedule: What to Choose and Order Before First Fix

An extension can stall halfway without anything going wrong with the building. It stalls on a decision. The doors can't be made because nobody signed off the sizes, and the tiler is booked while the tiles are still a shortlist.

A builder can only order what you've chosen. Anything you haven't chosen either stops the trade waiting for it or comes back later as a variation, priced at a point when you can't shop around.

The builder needs your product choices to price the work and plan when to order it, and most quotes leave the finishes as prime cost or provisional sums until you choose. The choices are yours. Unless the contract says otherwise, so are the deadlines. The specification schedule is where both live.

The Building Regulations points below are for England. Wales has its own Approved Documents and Scotland uses Technical Handbooks.

What goes on the schedule

Keep it in one spreadsheet, not a folder of brochures and email threads. Each line is one product, and every line answers the same questions:

The last three columns earn their place. "In the quote as" shows which choices still carry price risk. "Depends on" catches the chains: bifolds that can't be ordered until the floor build-up is settled, radiators that can't be sized until the heat loss is calculated.

Prime cost and provisional sums are open lines

Most quotes price the shell firmly and leave the finishes as allowances, of two kinds. A prime cost (PC) sum covers an item whose quantity is known but whose specification isn't: the floor area for tiles is fixed, the tile isn't chosen. A provisional sum covers work that hasn't been designed yet, such as a kitchen with no layout. Neither is a price. Each is replaced by the real cost once the choice is made.

The trap is the size of the allowance. A low PC sum makes a quote look cheaper without making it cheaper, because the difference comes back once you pick the product you actually wanted. Suppose one builder allows for a basic tile and another for the porcelain you've already shortlisted. The first quote wins on paper and is no cheaper once the tile is bought.

The builder's margin sits on top. Self-Build.co.uk puts the usual uplift for overheads and profit at 10-20% of the prime cost and says to agree the mechanism at tender. That's one publisher's indication, not a market rate, so get your builder's figure in writing before you sign. Too many open allowances leave the total open too, which defeats the point of a fixed price.

Every allowance is a schedule line marked "not decided". Turn as many as you can into named products before signing, and compare the rest by what each one buys. Getting quotes and comparing has the side-by-side method, and Rate My Quote shows what each quote includes, so the allowances and gaps are easier to spot.