How to set an extension budget before you design

Start with the amount available for the whole project

A construction quotation is only one part of an extension budget. Before deciding the size of the scheme, separate the total funds available from the amount you can spend on the building contract. Professional services, investigations, approvals, tax and temporary arrangements also need funding.

Write down what the finished project includes. Does the budget cover the kitchen, appliances, flooring through the existing rooms, landscaping and decorating? A quotation can look affordable because those items sit outside it. The answer is a clearer scope, not a more optimistic estimate.

Make a cost plan with separate headings

• Surveys, design and specialist investigations.
• Planning, technical design and statutory charges.
• Construction, including work to the existing house.
• Kitchen, bathrooms, fittings and finishes.
• Utility alterations and external works.
• Applicable VAT and other professional advice.
• Temporary accommodation, storage and finance costs where relevant.
• A contingency proportionate to the unresolved risks.

Use our published fee information and construction calculator as starting points. A calculator result is an early allowance, not a contractor's quotation or a promise of the final price. Confirm the assumptions, exclusions and fee scope before relying on it for a purchase or borrowing decision.

Reduce uncertainty before reducing quality

Drain positions, hidden structure and difficult site access can affect cost more than a change of paint finish. Where a risk is significant, a survey or targeted investigation may improve the budget more than repeatedly redrawing the kitchen. Ask which unknowns need investigation and which can reasonably remain an allowance.

Keep an option list. Separate essentials from features that can be simplified or postponed. Test changes against the whole design: removing a rooflight might save money but leave the centre of the house too dark, while a simpler structural opening could retain the main benefit at a lower cost.

Update the budget at each decision

Review the cost plan after feasibility, planning, technical design and tender returns. Record what has changed and who approved it. Once building starts, agree the price and programme effect of variations before instructing them wherever practicable.

Contact us with your address, priorities and total budget at the outset. An honest budget helps us propose a useful scope and compare options. It also gives you an earlier opportunity to phase the project or reconsider an element before paying for detailed work that cannot be funded.