Why the Cheapest Extension Quote May Cost You More

One builder quotes £58,000. Another quotes £52,000. The £52,000 builder looks cheaper—but is it really? Extension quotations can include different assumptions, allowances and exclusions. Unless you compare them properly, the lowest quotation can eventually become the most expensive.

Compare the same project

Give every builder the same drawings, specification and project information. This should include:

• Extension size and layout
• Roof type
• Windows, rooflights and external doors
• Kitchen or bathroom requirements
• Flooring and decorating
• Drainage and structural information
• Access and parking restrictions If Builder A has allowed for aluminium sliding doors and Builder B has assumed basic uPVC doors, their prices cannot be compared directly.

Check what the price includes

Imagine you receive these two quotations:

Builder A: £58,000

This includes:

• VAT
• Sliding doors
• Plumbing and electrical work
• Plastering and decorating

Builder B: £52,000

But this excludes:

• VAT
• Sliding doors
• Decorating Once you add approximately £10,400 VAT, £5,000 for the doors and £2,000 for decorating, Builder B could cost around £69,400. The apparently cheaper quotation is now over £11,000 more expensive.

Never compare only the number at the bottom of the quotation. Compare what you will receive for that number.

Look for missing costs

Before accepting a quotation, check whether it includes:

• VAT
• Scaffolding and skips
• Drainage alterations
• Structural steelwork
• Windows and external doors
• Plumbing and electrical work
• Kitchen or bathroom installation
• Flooring and decorating
• Making good to the existing house
• Building-control requirements Ordinary extensions to existing homes are generally subject to 20% VAT when the work is supplied by a VAT-registered contractor. Always ask whether VAT is included.

Understand provisional sums

A provisional sum is an allowance rather than a confirmed price. Builders may use provisional sums for:

• Foundations
• Drainage work
• Structural steel
• Kitchen products
• Electrical fittings If the actual cost is higher than the allowance, you will normally pay the difference. Ask what each provisional sum covers and whether it includes both materials and installation.

Read the exclusions

The exclusions can be more important than the total. Common exclusions include:

• Architect and structural-engineer fees
• Planning and building-control fees
• Party-wall surveyor costs
• Kitchen units and appliances
• Floor finishes
• Landscaping
• Unexpected ground or structural work Add these items to your wider project budget before deciding which quotation offers the best value.

Keep a contingency

Even a detailed quotation cannot predict every problem inside an existing building. Keep approximately 10–15% of the construction budget available for genuinely unexpected work. Do not spend this allowance on upgrades before construction begins.

Before choosing your builder

Check that:

• Every builder priced the same information
• VAT is clearly stated
• Allowances are realistic
• Exclusions are listed
• Important products are specified
• Payment stages are clear
• Changes must be agreed in writing A clear £60,000 quotation may be safer than an unclear £50,000 quotation.

The best quotation is not necessarily the cheapest. It is the one that gives you the clearest and most realistic route to completing your extension. Not sure whether a builder’s quotation looks realistic? Use the free BuildCostHub Extension Cost Calculator to create an initial UK cost range before comparing your quotations.