The Ultimate House Extension Budget Checklist (UK 2026)

Before You Plan Your Extension: Budget Checklist

Most homeowners focus on the builder's quote. In reality, the builder's quotation is only one part of your total project budget.

Use this checklist before starting your extension or loft conversion.

✅ 1. Finance

Have you considered:

• Savings available
• Mortgage or further advance
• Home improvement loan (if applicable)
• Mortgage interest costs
• Keeping an emergency fund

✅ 2. Professional Fees

Depending on your project, you may need:

• Architect or Architectural Technician
• Structural Engineer
• Measured Building Survey
• Topographical Survey
• Party Wall Surveyor (if applicable)
• Planning Consultant (if required)

✅ 3. Planning & Authority Fees

Remember to allow for:

• Planning Application (if required)
• Building Control Fees
• Thames Water Build Over Agreement (if required)
• Local Authority or Utility Company application fees (where applicable)

Typical guide prices (England):

• Planning Application: £500–£700+
• Building Control: £700–£1,500+
• Thames Water Build Over Agreement: £130–£585 Fees vary depending on your project and local authority.

✅ 4. Construction Costs

Your builder's quotation may include:

• Groundworks
• Foundations
• Structural Steel
• Brickwork
• Roofing
• Windows & Doors
• Plumbing & Heating
• Electrical Installation
• Plastering
• Decoration

Not sure what your project should cost? Try our free calculators: BuildCostHub | UK Construction Cost Calculator

✅ 5. Fixtures & Finishes

These are often underestimated. Remember to budget for:

• Kitchen Units
• Worktops
• Kitchen Appliances
• Bathroom Suites
• Wall & Floor Tiles
• Flooring
• Internal Doors
• Light Fittings
• Built-in Wardrobes
• Decorating
• Landscaping & External Works

✅ 6. Utilities & Service Alterations

Some projects may also require:

• Drainage alterations
• Manhole relocation
• Water supply alterations
• Gas pipe alterations
• Electricity meter or supply relocation
• EV charger relocation (if applicable)

These costs don't apply to every project, but when they do, they can add thousands of pounds to your budget.

✅ 7. Living Costs During Construction

Depending on the scale of your project, remember to consider:

• Temporary accommodation
• Storage
• Additional childcare or pet care
• Professional cleaning after completion

✅ 8. Contingency

Always allow for unexpected costs.

• Minimum contingency: 10%
• Older properties or complex renovations: 15–20%

A contingency fund can prevent unexpected issues from becoming expensive problems.

Final Check

Before you start your project, ask yourself:

• Do I know my total project budget?
• Have I included professional and statutory fees?
• Have I budgeted for fixtures and finishes?
• Have I allowed for utilities and service alterations?
• Have I included a contingency?
• Do I know whether extending is the best financial option?

How BuildCostHub Can Help

BuildCostHub helps homeowners make better building decisions with confidence. Start by estimating your construction costs with our free calculators, then compare your options before committing to a design.

Coming Soon: Our Move or Improve decision tool will help homeowners compare extending, loft conversions and moving house based on both construction costs and potential property value.