Two builders quote the same UK extension. One returns a single number. The other returns a rate, a margin and a promise of receipts. These are not two prices for one job. They are two different allocations of risk, and fixed price vs cost plus is the one contract decision a homeowner makes before anybody touches the building.
A fixed price — in UK domestic work usually a lump sum — means the builder carries the risk that the work costs more than they expected. They have priced that risk, and it is inside the number whether or not anything goes wrong. Cost plus means you reimburse actual labour, materials, plant and subcontractor costs and pay an agreed percentage or fee on top. The risk premium disappears, and the risk lands on you.
Neither model is dishonest and neither is safer in the abstract. What makes one safe and the other expensive is a document most UK homeowners have never heard of, and it is published by the same body that writes the contracts used on commercial sites across the country.
A fixed price is not a promise about cost. It is a decision about who pays when the cost turns out to be wrong.
What follows is what each model genuinely commits your builder to, where the JCT Home Owner contracts fit, what UK consumer law hands you when the paperwork is thin, and how to decide.
What does fixed price vs cost plus mean on a UK building contract
A fixed price is one figure for a defined scope. Any cost the builder failed to foresee inside that scope is theirs to absorb. Any work outside it is a variation, and that is yours. The whole argument on a fixed-price job is therefore about where the boundary of the scope sits, which is why the scope document matters more than the price.
Cost plus, also met as cost reimbursable or prime cost, works the other way. The builder charges what the job actually costs and adds an agreed percentage or a fixed management fee. There is no risk premium because the builder is carrying no risk. There is also no ceiling unless you write one in.
Two words decide which one you are holding. In the UK a quote is a fixed offer the builder can be held to. An estimate is an opinion about likely cost and binds nobody. Builders use the words loosely and homeowners read them hopefully, which is the first place a fixed price stops being fixed. Learning to read a renovation quote properly is the prerequisite for this decision, not an optional extra.
What does a fixed price actually fix
Less than the name suggests. A fixed price fixes the measured portion of a defined scope, priced on the information available on the day it was measured. Three things sit outside that.
The first is provisional and prime cost sums. These are placeholders written into the price for work or goods that could not be measured, and they are replaced later by a valuation of what was actually done. A quote carrying a large share of its value in provisional sums is a fixed price in name only.
The second is variations — anything you change, and anything the drawings did not show. The third is VAT. Check whether the figure is stated inclusive or exclusive, and whether the builder is registered at all: VAT registration is compulsory once taxable turnover passes £90,000, so a price from an unregistered sole trader and a price from an established firm are not comparable at face value. Twenty per cent is not a rounding error on a £60,000 project.
None of this makes fixed price the wrong choice. It makes the scope document the thing that carries the protection, because a price is only as firm as the description behind it.
Get your own number before either quote arrives
The free UK Renovation Cost Calculator gives you a trade-by-trade estimate in under 5 minutes — before your first trade conversation. Without your own baseline you cannot tell a keen fixed price from an underpriced one.
When is cost plus the safer choice
When the scope genuinely cannot be defined. Opening up unknown structure behind lath and plaster, working on a listed building, repairing fire or water damage, or running a phased refurbishment where each stage informs the next — nobody can write a scope precise enough to price.
Forcing a fixed price onto an undefinable scope produces one of two outcomes, and both cost you. Either the builder loads the number with contingency you pay whether or not it is needed, or the quote fills with provisional sums, which is cost plus in disguise without the transparency that makes cost plus tolerable.
The difference between cost plus working and cost plus running away is entirely a matter of controls. Open-book means you see the same paperwork the builder sees, and it has to be written into the contract before work starts rather than requested once the invoices look wrong.
Open book. Named labour rates, timesheets and every supplier invoice, produced with each application for payment rather than on request.
A ceiling. A guaranteed maximum price or a target cost with an agreed share of any overrun, so the arrangement has a top.
An approval threshold. A figure above which nothing is ordered or committed without your written sign-off, named in the contract.
What is the JCT Homeowner contract and why does it change the answer
The Joint Contracts Tribunal publishes the standard building contracts used across UK construction, and most homeowners assume its forms are for commercial projects only. They are not. JCT publishes a range of Home Owner Contracts written for consumers, in two parts — Arrangements and Conditions — in deliberately plain language drafted to comply with consumer legislation.
The 2021 range covers the three situations a homeowner is actually in. HO/B is the building contract for a home owner or occupier who has not appointed a consultant to oversee the work. HO/C together with HO/CA covers the case where you have appointed one. The Home Repair and Maintenance Contract, HORM, covers small repair and maintenance work and is published by JCT as a free download.
Here is why the choice of form decides the pricing model rather than following it. The homeowner forms are built for simplicity: they provide for a single payment at the end or payments at agreed stages, and they leave out the payment and pay-less notice machinery carried by the commercial contracts. That simplicity is a good fit for a lump sum against a defined scope. It gives you almost none of the valuation and open-book machinery that a cost plus arrangement depends on.
So the sequence runs one way. If you have no consultant administering the work, HO/B plus a fixed price against a tight scope is the arrangement the paperwork is designed to support. If the scope cannot be defined and you want cost plus, you need either a consultant administering the contract or the open-book controls above written in explicitly. Choosing cost plus on a form that never anticipated it is how homeowners end up arguing about invoices with nothing to argue from.
One more thing the four-nation reader needs. The JCT forms are drafted for England and Wales. In Scotland the equivalents come from the Scottish Building Contracts Committee, and in Northern Ireland JCT contracts are used with the Northern Ireland adaptation schedules issued through the Royal Society of Ulster Architects. And whoever's contract arrives by email — a Federation of Master Builders member's own form, or the builder's — read whose interests drafted it before you sign it.
What does UK law give you when the contract is silent
A floor, not a plan. Under section 51 of the Consumer Rights Act 2015, where a contract to supply a service does not expressly fix a price and does not say how the price is to be fixed, the contract is treated as including a term that you must pay a reasonable price and no more. What is reasonable is a question of fact. Section 52 does the same for time where no completion date was agreed.
That protection is real and it is also expensive to rely on, because establishing what was reasonable happens after the money has been spent and usually after the relationship has broken down. It is a backstop for the homeowner who never agreed a price. It is not a substitute for agreeing one.
There is a second protection worth knowing before the builder puts a pen in your hand. A contract agreed in your home is an off-premises contract under the Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013. Regulations 29 and 30 give a right to cancel a service contract within 14 days of the day it was entered into, and regulation 31 extends that period by up to 12 months if the trader failed to give you the required cancellation information. If you ask for work to start inside the cancellation period you can become liable for what has been supplied, so the two decisions are linked.
Both apply UK-wide, and both are reasons to take the contract away and read it rather than signing at the kitchen worktop.
How should a homeowner choose between fixed price and cost plus
The choice is an output, not an opinion. Work through the sequence and the answer produces itself.
- Write the scope before you ask for a price. A written scope of works is what makes competing quotes comparable and what makes a fixed price genuinely fixed.
- Count the unknowns you can close. Trial holes, a drainage survey and opening up a section of ceiling cost a few hundred pounds and convert guesses into measurable work.
- Total the provisional and prime cost sums on every quote. If a large share of the price is provisional, you are being offered cost plus with a fixed-price label on the front.
- Decide who administers the contract. Cost plus without a consultant means you are valuing the work yourself every month, which is a real job with a real time cost.
- Match the contract form to the model. HO/B suits a lump sum against a tight scope, and anything open-book needs the controls written in before work starts.
- Set the money mechanics in writing. Stage payment points tied to verified milestones, a written variation procedure, a retention held against the snagging list, and named responsibility for Building Regulations approval sign-off.
Every item on that list belongs to Phase 6 of The 12-Phase System, the contract review every later phase inherits. Reach Phase 6 with a finished scope and you can safely take a fixed price. Reach it with an unfinished one and you are choosing between two ways of paying for that gap, of which the fixed price is usually the dearer. It is also the most common reason UK renovations go over budget without anybody doing anything wrong.
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Frequently asked questions
Is fixed price or cost plus better for a UK renovation?
Neither is better in the abstract. Fixed price suits a scope that can be fully described, because the builder carries the risk and prices it once. Cost plus suits a scope that genuinely cannot be described, provided you have open-book access, a ceiling or target cost, and a written approval threshold. The deciding question is whether your scope document is finished.
What is a JCT Homeowner contract and where do you get one?
It is a building contract published by the Joint Contracts Tribunal specifically for consumers, written in plain language in two parts, Arrangements and Conditions. The 2021 range includes HO/B for a home owner without a consultant, HO/C with HO/CA where a consultant is appointed, and the free Home Repair and Maintenance Contract for small works. They are obtained from the JCT online store, and the pack includes copies for both parties.
Does a fixed price quote include VAT in the UK?
Only if it says so. Always confirm whether the figure is stated inclusive or exclusive of VAT and whether the builder is VAT-registered, since registration is compulsory once taxable turnover passes £90,000. An unregistered sole trader charges no VAT, so their price is not directly comparable with a registered firm's until you have adjusted for it.
Can I cancel a building contract I signed at home?
Usually yes. A contract agreed in your home is an off-premises contract under the Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013, which give a 14-day right to cancel running from the day the contract was entered into. If the trader did not give you the required information about that right, the period can extend by up to 12 months. Asking for work to begin inside the period can make you liable for what has been supplied.
What happens if we never agreed a price at all?
Section 51 of the Consumer Rights Act 2015 treats the contract as including a term that you pay a reasonable price and no more, where no price was expressly fixed and no method of fixing one was agreed. What counts as reasonable is a question of fact. It is a genuine protection, but proving it happens after the money has gone, which is why agreeing the price in writing is cheaper.
Are UK building contracts the same in Scotland and Northern Ireland?
No. JCT forms are drafted for England and Wales. Scottish projects use the Scottish Building Contracts Committee suite, and in Northern Ireland JCT contracts are used with Northern Ireland adaptation schedules issued through the Royal Society of Ulster Architects. The approval regimes differ too: Building Regulations approval in England and Wales, a building warrant in Scotland, and district council building control in Northern Ireland.