Two Extensions With the Same Floor Area Can Have Significantly Different Costs.
A quotation is only useful when the scope behind the total is clear. Two quotations with different totals cannot be properly compared unless the customer also compares the work, specifications, allowances, exclusions and responsibilities included within each one.
Why Square-Metre Prices Can Be Misleading
Square-metre pricing is often used as a quick rule of thumb, but it rarely reflects the reality of a bespoke building project. They can provide an early broad indication only where the assumptions and level of finish are clearly stated.
Structural Alterations
They often exclude the cost of knock-throughs and steelwork to connect the existing home.
Kitchens & Bathrooms
They typically exclude the supply and installation of kitchens, sanitaryware and tiling.
Glazing & Doors
They often assume standard windows, excluding large bifolds or roof lanterns.
Professional Fees
They rarely include architectural, structural engineering or Building Control fees.
Drainage & Services
They may exclude moving manholes, drains or altering gas and water mains.
Ground Conditions
They usually assume uncomplicated, flat ground with standard foundation requirements.
Standards of Completion
They may describe a plastered shell rather than a fully decorated, turn-key finish.
VAT
They often exclude VAT, which adds 20% to the final construction cost.
Why Two Similar-Sized Extensions Can Cost Differently
Example Extension A
- Straightforward footprint
- Standard glazing
- Limited structural alterations
- Existing services nearby
- Basic internal specification
Example Extension B
- Complex footprint
- Large structural knock-through
- Extensive glazing
- Drainage alterations
- Higher internal specification
This is an illustrative comparison rather than a quotation or price example.
The Main Factors Affecting Extension Costs
Size and Footprint
The overall dimensions directly impact material quantities, labour time, and waste removal.
Single or Two-Storey
A two-storey extension shares the same foundation and roof footprint, changing the cost dynamic.
Ground Conditions
Poor soil or nearby trees can require deeper or piled foundations, increasing costs early on.
Drainage & Services
Moving manholes or altering gas and water mains adds complexity to the groundworks phase.
Structural Steels
Connecting the new space requires removing walls. The size and weight of steel beams affect the price.
Roof Design
A flat roof is generally more cost-effective. Adding roof lanterns increases material costs.
Windows & Doors
Large aluminium bifold doors or bespoke feature windows are a major cost variable.
Electrical Work
The number of circuits and whether your existing consumer unit requires upgrading influences costs.
Plumbing & Heating
Extending heating or adding underfloor heating requires careful planning and affects the budget.
Kitchen or Bathroom
The chosen products, appliances, and sanitaryware can drastically alter the final project cost.
Internal Finishes
The extent of plastering, new doors, skirting, and the quality of timber contribute to finishing costs.
Flooring & Decoration
High-end tiles or engineered wood cost significantly more to fit than standard laminate.
Access & Logistics
Restricted access means materials may need to be moved by hand, increasing labour costs.
Waste & Protection
The volume of skips required and materials needed to protect your home must be accounted for.
Professional Fees
Architects, engineers, and Building Control inspections are necessary costs outside construction.
VAT & Contingency
Always check if a quotation includes VAT. A sensible contingency should also be planned.
Shell-Only Versus a Complete Finished Extension
When discussing costs, it is vital to understand what level of finish is being quoted. These labels are not universal and the written inclusions matter more than the label.
| Description | Typical inclusions | Still needs confirming |
|---|---|---|
| Structural shell | Foundations, walls, and roof structure only. | Windows, doors, internal works, services. |
| Weather-tight shell | As above, plus roof coverings, windows, and external doors. | Plastering, electrics, plumbing, finishes. |
| Plastered finish | Fully constructed, insulated, first-fixed, and plastered. | Second-fix carpentry, decorating, flooring. |
| Ready for decoration | Second-fix carpentry and electricals complete. | Paint, flooring, final fixtures. |
| Fully finished extension | Complete turn-key finish, including kitchens and decoration. | None (assuming all scope is agreed). |
Why Drawings and Specifications Improve the Quotation
A dependable quotation requires detailed information. The more information provided, the more accurate the price will be.
What Should Be Included When Comparing Extension Quotations?
- Scope of work clearly defined
- VAT status confirmed
- Structural work included
- Windows and doors specified
- Electrics and plumbing covered
- Internal finishes detailed
- Professional fees accounted for
- Waste removal included
- Site protection planned
- Programme schedule estimated
- Payment stages agreed
- Exclusions clearly listed
- Allowances identified
- Guarantee terms provided
- Insurance cover confirmed
Should You Keep a Contingency?
Unforeseen issues can occur when working with an existing building. The appropriate contingency depends on the information available, the property and the customer’s circumstances. A contingency fund ensures that unexpected discoveries do not derail the project.
How Milano Approaches Extension Quotations
Milano aims to prepare quotations around the actual proposed work rather than relying solely on a headline square-metre figure. Drawings, structural information, specifications and a site visit help us identify the known scope, allowances and exclusions more clearly.
The cheapest headline figure is not always the lowest final cost. The important question is what the quotation genuinely includes.