Additional Costs to Consider When Planning a Home Extension
When budgeting for a home extension, it’s easy to focus on design fees and build costs. However, there are a number of additional costs that may apply depending on your project.
Not every extension will require these, but when they do apply, they are essential for keeping your project compliant and moving forward smoothly.
At Plan It UK, we aim to keep everything transparent so there are no surprises later. Here are the most common additional costs, why they matter, and how we help manage them.
1. Party Wall Awards
If your extension is built close to a shared boundary, you will usually need to serve a Party Wall Notice.
If your neighbour consents, there is no additional cost. However, if they do not consent, or if the neighbouring property is owned by a council or housing association, a Party Wall Award will be required.
This is a legal agreement prepared by a surveyor and typically costs around £950 + VAT per neighbour.
Good to know: This is a standard part of the process and does not indicate a problem. It simply ensures all parties are protected. We can manage this process for you if needed.
2. SAP Calculations
If the increase in glazing exceeds 25% of the extension’s floor area, SAP calculations are required to demonstrate compliance with Building Regulations.
We can arrange SAP calculations for £125 + VAT.
In some cases, the results may require minor adjustments to your Building Regulations drawings. If this happens, allow around three additional working days.
3. Tree Surveys
For projects involving new foundations, a soil test is typically carried out.
If the results show a plasticity index of 10% or higher, a tree survey will be required to assess nearby trees and their potential impact.
This survey identifies all trees within 30 metres of the proposed foundations and evaluates their water demand.
We can arrange this through a specialist for £280 + VAT and will confirm if it is required once soil test results are available.
4. Drainage Surveys
For ground-floor extensions, we usually carry out a basic visual drainage check.
If further detail is needed, a CCTV and line tracing survey may be required to accurately map your drainage system.
This typically costs around £350 + VAT, and we can organise the process for you.
5. Specialist Planning Reports
Some planning applications require additional reports to meet local authority requirements.
These may include:
- Heritage Statements for listed buildings or conservation areas
- Ecology Reports if protected species may be affected
- Biodiversity Net Gain Assessments where required
- Flood Risk Assessments for properties in flood zones
Not all projects require these, which is why they are not included in initial quotes. However, if they are needed, they are essential for securing approval.
We will always flag these early and can coordinate them on your behalf to keep things straightforward.
The Bottom Line
These additional costs do not apply to every project, but when they do, they play an important role in ensuring your extension is safe, compliant, and ready to build.
At Plan It UK, we highlight these as early as possible so you can plan your budget with confidence and avoid unexpected costs.
At a Glance: Typical Additional Costs
- Party Wall Award: £950 + VAT per neighbour
- SAP Calculations: £125 + VAT
- Tree Survey: £280 + VAT (if required)
- Drainage Survey: approximately £350 + VAT
- Specialist Reports: cost varies depending on requirement
Need a Clear Breakdown for Your Project?
If you are just starting your extension and want a clear picture of the costs involved, we can help.
Get in touch or book a consultation and we will walk you through the process step by step.
