What is a cost plan, and do I need one?

If you are planning an extension, renovation or new home, you want to know whether you can afford it before you become attached to the design. That sounds obvious. Yet I regularly see the cost conversation left until most of the decisions have already been made.

A cost plan brings that conversation forward. It helps you and your architect understand what the design is likely to cost, where the money is going and what your choices mean for the budget.

What is a cost plan?

For homeowners, we describe our Elemental Cost Plan as a design cost check. It is an independent estimate prepared by a quantity surveyor from your drawings, brief and the information available at that stage.

“Elemental” means we break the cost down into parts of the building: foundations, walls, roof, windows, finishes and services. You can see the effect of changing a particular part of the design, instead of trying to make sense of one lump sum.

It is an estimate, not an offer from a builder to carry out the work. Its value is in helping you make informed decisions while you still have options.

We are looking ahead to the likely cost of completing the project. That means making reasonable allowances for design development and identified risks, rather than assuming that everything missing from the current drawings will cost nothing. Completed-project costs and tender data help inform that judgement; the assumptions still need to be explained.

Start with what you want to achieve

The starting point is your brief: what you need from your home, what matters most to you and what you are willing and able to spend. Then we test whether those things fit together.

My job is not to tell you what you can or cannot have. If a particular design is worth spending more on, that is your decision.

What matters is that you understand the cost and make that decision deliberately.

If the budget cannot move, we need to explore the options while the design still can.

That is why I prefer cost advice during the early design stages. Cutting things out after the builders’ prices come back too high is a much harder conversation.

What should a cost plan include?

A useful cost plan explains the basis of its figures as well as the total. It should make clear:

• The estimated cost of each building element, with quantities and rates at a level appropriate to the drawings.

• The assumed specification and allowances for choices you have not yet made, such as glazing, kitchens and finishes.

• What is included, what is excluded and where information is missing.

• How location, access and the nature of the work have been considered.

• Separate allowances for uncertainty and inflation, with the treatment of VAT and professional fees clearly identified.

Check the overall project budget as well as the building cost. Fees, surveys, statutory charges, furniture or temporary accommodation may need funding outside the building works. They do not disappear because they are outside a builder’s price.

What does a cost plan look like?

These extracts show how we present an Elemental Cost Plan for different types of home project. The sample reports use fictional project details and historical costs to illustrate the format. They are not current prices for your project.

Example 1: a new-build house

The budget summary brings the main work packages together and shows the allowances for contingency and inflation. Read it alongside the stated scope: this particular sample excludes professional fees and loose furniture.

New-build ECP: the overall budget summary, with its inclusions and exclusions. View the budget extract at full size.

The detailed pages then show how that budget is built up. You can see the description, quantity, unit, estimated rate and total for individual items.

New-build ECP: demolition and foundation costs within the wider plan. View the detailed extract at full size.

Example 2: refurbishment, extension and loft conversion

For a project combining refurbishment, an extension and a loft conversion, we typically organise the cost plan by these defined areas of work. You can see the cost of the internal works, rear extension and loft conversion separately. Each section then has its own elemental breakdown, so you can understand both the cost of that part of the project and what makes it up.

The summary below brings together three section totals from the sample ECP. These are historical construction costs for selected areas of work; other work and the wider project allowances are dealt with separately in the report.

Selected construction section totals summarised from the sample ECP. Each area has its own detailed breakdown. View the work-area summary at full size.

This is where the cost plan becomes especially useful for your decisions. You can weigh up what each part of the project will give you against what it is likely to cost, then decide where you want to spend. You and your architect can test changes to the size or specification of one area, prioritise the work that matters most, or explore carrying out some work later. We then check the cost of those options, including any shared work and knock-on effects.

In each case, the cost plan already contains our estimated rates and costs. A Schedule of Works gives builders the work to price. You can see an example of that format in the linked guide.

Use it to compare choices

The useful question is often: what difference would this option make? A smaller extension, a different roof or a change in layout can affect the budget far more than small savings on finishes later.

Once the main approach is agreed, allowances for items such as kitchens and bathrooms give you something to work with. If you choose to spend more in one area, you can see whether that means spending less elsewhere or increasing the overall budget.

This works best as a conversation between you, your designer and the person advising on costs. A report left in a drawer will not keep pace with a developing design.

Why the cost plan needs updating

I keep returning to a project we worked on a few years ago and use it as an example. The early cost plan supported a budget of about £750,000. During design development, a proposal to replace and move the staircase was taken forward. It was a good design idea, but it also meant additional structural work and changes to the layout.

When we were asked to update the cost plan, the staircase and associated work added about £200,000. By then, the client and architect were already invested in the design. The difficulty was not just finding the money; it was the disappointment of having to reconsider something they wanted.

The lesson is to check the cost before committing to a significant change.

Review the plan at key design decisions, before planning submission where relevant, and as important specifications are agreed. If the brief changes, revisit the budget too.

How reliable is an Elemental Cost Plan?

Our Elemental Cost Plans are built on real project data and current market rates. The aim is to give you a sound estimate of the full likely cost of completing your project, including reasonable allowances for work and decisions still to be developed.

We prepare close to 200 ECPs a year. When those projects go to contractors, we generally find our figures are reasonably close to the prices reputable builders return for the full scope of work. We track those comparisons to check our accuracy and understand how the market is changing.

The comparison needs to cover the same work. A low headline quote that leaves out parts of the project can make the budget look more comfortable than it really is. We want you to understand the likely end cost from the outset. I explain this further in Why ‘We’ll Just Ask a Contractor’ Is Usually a Mistake.

An ECP cannot guarantee an individual builder’s price, and early designs need more assumptions than developed ones. We make those assumptions clear so you can see what the estimate is based on. You have a realistic, evidence-based budget to work with as you develop the design and make decisions.

Do you need one yet?

If you have early drawings and are comparing options, developing a planning application or committing to more detailed design, a cost plan will help you make the next decision with better information.

If you have no drawings yet, you can still test whether your ambitions and budget broadly fit. Start with a clear brief and an initial budget discussion. Our free calculator gives an indicative building-cost range; the Budget Reality Check, at £195 including VAT, includes a quantity surveyor’s review, a 30-minute online call and a written action list. It is not a detailed cost plan.

Design cost check fees start at £595 including VAT. We confirm the scope and fixed fee before you pay. The £195 Reality Check fee is credited if you book a design cost check or builder pricing pack for the same project within three months, subject to the service terms.

When the design is ready for builders to price, a Schedule of Works helps give them a common basis for their quotes. If you are unsure which document you need now, read our guide to choosing between a builder’s quote, cost plan and Schedule of Works.

Choose your next step

Try the free budget calculator, or see our services for homeowners if you already have drawings. The aim is to get the right level of advice for the decision in front of you.