What else should I budget for before I build?

When you’re budgeting for a home extension, it’s easy to focus on the design fees and the build cost, but there are a few other costs you might need to allow for along the way. Not every project requires these, but if they do apply, they’re usually essential for keeping your project compliant and on track.

Our goal is to keep things as transparent as possible so that you don’t run into surprises later. Here are the most common additional costs we see on projects, why they matter, and how we can help manage them for you:

1. Party Wall Awards

If your extension is built up to (or very near) a shared boundary, you’ll usually need to serve a Party Wall Notice to your neighbours. If they consent to the notice, there’s no additional cost.

However, if your neighbour doesn’t consent – or if the neighbouring property is council or housing association owned – a Party Wall Award will be required. This is a legal agreement prepared by a surveyor and typically costs £950 + VAT per neighbour.

Good to know: The need for a Party Wall Award isn’t a bad sign – it simply protects everyone involved and allows you to proceed with your build lawfully. We can coordinate this process for you if it becomes necessary.

2. SAP Calculations

If the net increase in glazing for your extension is more than 25% of the new floor area (which is the case for most projects), you’ll need SAP calculations to show compliance with building regulations.

We can arrange SAP calculations for £125 + VAT. Very occasionally, these calculations may require adjustments to your Building Regulations package – if so, allow an extra 3 working days for us to make the changes.

3. Tree Surveys

For projects where the ground needs to be dug for new foundations, we always conduct a soil test. If the results show a plasticity index of 10% or higher, you’ll need a tree survey to identify all trees within 30m of the proposed foundations and assess their water demand.

We can arrange this through a trusted specialist for £280 + VAT and will confirm whether it’s needed once we have the lab results.

4. Drainage Surveys

For ground-floor projects, we usually carry out a basic visual drain inspection as part of our process. However, sometimes this isn’t enough to confirm feasibility.

If we need more information, we’ll recommend a CCTV + line tracing drainage survey to map your existing drainage accurately. This service typically costs around £350 + VAT, and we can coordinate everything for you.

5. Specialist Reports for Planning Applications

Some planning applications need extra supporting information to address the local authority’s requirements. These can include:

• Heritage Statements – if your property is listed or in a conservation area

• Ecology Reports – if protected species might be affected by your project

• Biodiversity Net Gain Assessments – now required on some sites to demonstrate a measurable improvement to biodiversity

• Flood Risk Assessments – if your property is in a flood zone

Not every project will need these, which is why they aren’t included in our initial quote. But when they are required, they’re a crucial part of getting your application approved. We’ll always let you know as early as possible if we think any specialist reports are likely to be required, and we can coordinate most of these services on your behalf to keep things simple.

The Bottom Line

These extra costs don’t apply to every project, but when they do, they’re an important step to ensure your extension is safe, lawful, and buildable. At Plan It, we’ll always flag these items as early as possible so you can budget accordingly: no hidden surprises.

If you’re just starting your project and want a clear picture of what to expect, book a consultation with us and we’ll walk you through the entire process step by step.

At a Glance: Typical Additional Costs

Here’s a quick summary of the costs mentioned above:

• Party Wall Award: £950 + VAT per neighbour

• SAP Calculations: £125 + VAT

• Tree Survey: £280 + VAT (if soil plasticity index ≥ 10%)

• CCTV + Line Tracing Drainage Survey: ~£350 + VAT

• Specialist Planning Reports: Price varies depending on report type (we’ll confirm early and coordinate if needed)