Planning a House Extension: What You Really Need to Consider in 2025

Planning a House Extension: What you need to consider in terms of build and cost

When preparing for your home extension, cost and time are the interwoven dominating factors. Whilst it’s never easy to give a cost range typical build prices in 2025 range from £1,200 to £3,000 per square metre, depending on:

The type of extension (single-storey, double-storey, side), the location (London and the South East will probably command higher rates) and the quality of finish for all round décor, especially kitchens, bathrooms and bespoke finishes.

At the beginning of the work consider the logistics such as site access and ground conditions. How much labour is needed to level the site? What are the logistical problems?

But cost isn’t just about the works and the finishes. The whole project is interwoven.

Here are the 4 major areas that in our experience you need to think about before you start.

1) Structural & Technical Considerations (Building Regulations Part A & L)

A basic understanding of the major technical issues that need to be addressed during your build are highlighted under Parts A and L of the Building Regulations.

The framework and foundations of your extension are the key structural areas. Building Regulations Part A, “covers the loadings on a building, and the construction of the structural elements including the foundations, walls, floors, roofs and chimneys.”

Once the site is cleared, the foundations need to be excavated: Soil type, existing drainage, and proximity to trees or other buildings will determine the kind of foundation, strip, pile, or raft, you require.

Most extensions require steel beams or lintels—especially if you're opening up load-bearing walls or creating large spans for open-plan layouts. (This is often the case when exterior walls are knocked through). Beams must be sized and specified by a qualified structural engineer in order to be suitable for the appropriate loading.

Insulation & Thermal Performance: Compliance with Part L of Building Regulations.

Legislative amendments and heightened energy awareness have, in recent years, led to advances in the need to conserve fuel and power.

As the building takes shape insulation and thermal performance become key factors. When understanding U-values, the measurement of how well a material or building retains heat is crucial.

The lower the U-value, the better the insulation, the less heat you lose.

Specific minimum U-values for walls, floors, roofs, and windows must be met. Rigid foam is the industry’s favourite insulative material, laid to specified thicknesses around walls and on floors. Rockwool wall is usually run between ceiling joists to stop heat rising through the ceiling.

Windows and doors also impact energy efficiency, ventilation, and the overall structure. If you're aiming for high-performance builds, triple glazing might be on your to-do list.

2) Regulatory & Professional Fees

Consider costs for: Building Control specific plans and drawings, Building control submission fees, Structural engineer, Tree survey, Professional reports such as sound testing, and SAP assessments.

In the ideal world your extension should be a permitted development, but this may not always be the case. Conservation areas and listed buildings will require planning permission, so will double storey extensions and indeed anything to the front of your property as well as anything larger than the permitted volumes.

3) Unforeseen impact of the extension

Garden Reduction: Losing outdoor space can affect your resale value and lifestyle. Budget for landscaping to integrate the new structure.

Visual Cohesion: A high-spec extension can make the rest of the house feel dated. Will you need to refurbish adjoining rooms to match or even the whole house?

Living Through the Build: Staying on-site during construction is rarely as simple as it sounds. Especially if you’re project managing yourself (we really don’t recommend you do this unless you’ve had previous experience, see below.)

Bear in mind all the plant will be run from your power supply, so expect higher bills in terms of electricity and water.
• Dust infiltration despite barriers
• Loss of heating and hot water
• Noise, disruption, and limited privacy
• Temporary cooking and washing arrangements

4) Project Management: Think Twice

Many homeowners consider managing the build themselves to save on fees (typically 10–20%). But unless you have experience in construction sequencing, procurement, and contractor coordination, it’s a false economy.

We have been instructed in the middle of quite a few projects, when relations have become strained between both builders and clients. It’s understandable. If you don’t have full confidence in your builder, you shouldn’t have contracted them in the first place.

There are various reasons why a job runs over time, if it does, that’s going to be frustrating, but it’s also frustrating for your builder if the fault is down to your inexperience.

Make sure you’re able to interpret structural drawings, schedule trades in logical order, be pleasant at all times, and make sure you are one step ahead.

Without expertise, you risk delays, strained relationships with tradespeople and ultimately budget overruns.

Final Thoughts

• VAT: Home extensions are not zero-rated. You’ll pay 20% VAT on materials, labour, and professional services.
• Two Storeys vs One: Since foundations and roofing are required either way, adding a second storey often improves cost-efficiency per square metre, but you’ll need planning permission.
• Contingency: Always allow 10–15% for unforeseen issues—especially with older properties.

Final Final Thoughts

Building an extension is a major undertaking. It’s not just about adding space—it’s about integrating structure, systems, and style into your existing home. Get the technical foundations right, and the rest will follow. That means getting your building regulation application right from the start. Of course, from the perspective of building control, a good set of plans, stops rejection and diminishes the need for conditions.
A good application with drawings specific to your actual project, will enable the builder to price accurately, negating the unforeseen costs experience at the end of the job and highlighting all the issues from the project management aspect­—we’re always happy to discuss this with you as part of acting for you.