Budgeting for a Home Extension: Hidden Costs to Consider

When planning an extension in Newton Abbot, Torbay, or anywhere across South Devon, the initial quote for “bricks and mortar” is rarely the final figure. In 2026, building costs in the South West typically range from £1,800 to £3,000 per square metre, but hidden expenses can add a further 15% to 30% to your headline budget.

At V3 Build, we believe in financial transparency. To help you plan effectively, here are the most common “hidden” costs that homeowners often overlook.

1. Professional and Statutory Fees

Before a spade hits the ground, you will likely encounter a series of necessary professional costs. These are essential for ensuring your project is legal, safe, and built to modern standards.

• Architectural Drawings: While some builders offer design services, a full set of technical drawings can cost between 3% and 15% of the total build cost.

• Structural Engineer: You will almost certainly need a structural engineer to provide calculations for steel beams (RSJs) or foundation depth, which typically cost between £500 and £2,500.

• Planning and Building Control: Application fees for planning permission in Devon are approximately £258, while Building Control inspection fees—which ensure the work meets safety regulations—can range from £600 to £1,500 depending on the project size.

2. Groundworks and Site Surprises

The most unpredictable costs are often found beneath the surface. Once excavation begins, several factors can trigger immediate budget increases.

• Soil Quality and Foundations: If the ground is found to be unstable or “shrinkable” clay, you may require deeper or specialist foundations, such as piling.

• Drainage and Utilities: Moving a public sewer or a gas main is an expensive process that requires permission from the utility provider (such as South West Water). Even simple drain diversions can add thousands to the groundwork phase.

• Tree Roots: Proximity to large trees may necessitate root barriers or specific foundation designs to prevent future subsidence.

3. Party Wall Agreements

If you live in a semi-detached or terraced home in Exeter or Plymouth and your extension involves work on or near a shared boundary, you are legally required to serve a Party Wall Notice.

If your neighbor dissents or requests a formal “Award,” you will need to pay for a Party Wall Surveyor. Fees for this generally start at £700 per neighbor, and if multiple surveyors are required, this can quickly escalate to over £2,000.

4. Upgrading Existing Systems

An extension adds more volume to your home, which can put a strain on your current infrastructure.

• Boiler Capacity: A boiler that was perfectly adequate for a three-bedroom house may not have the “head” to heat a new large extension and additional radiators. An upgrade can cost between £2,500 and £4,000.

• Electrical Consumer Units: Older properties may need a full upgrade of the fuse board (consumer unit) to accommodate new circuits and meet current safety regulations.

5. Finishing Touches and VAT

It is a common pitfall to budget for the “shell” of the build but forget the costs of turning it into a living space.

• Landscaping: Construction work inevitably damages your garden. Factoring in the cost of new paving, turf, or fencing is essential for a finished look.

• VAT: Unless you are building a brand-new home, most renovation and extension work is subject to 20% VAT. Always check if your quotes include or exclude this, as it is a significant addition to the total.

• The 10% Contingency: We always recommend holding back a 10% to 15% “emergency fund.” This is not for extra features, but specifically for the “unknowns” that arise during the construction process.

Strategic Planning with V3 Build

With over 25 years of experience in Newton Abbot and the surrounding areas, V3 Build helps homeowners navigate these complexities. We provide detailed, transparent quotes that aim to identify as many of these “hidden” costs as possible during the initial survey.