Key Takeaway
Most of the problems that cost homeowners money on an extension could have been spotted before a single drawing was produced.
Most of the problems that cost homeowners money on an extension could have been spotted before a single drawing was produced. A feasibility study is that first step. It is an honest look at whether what you want is likely to be allowed, buildable and affordable on your property.
What is an extension feasibility study?
A feasibility study tests your idea against the planning rules, your site's constraints, your neighbours and boundaries, drainage, structure and budget. It will not give you a final design. It stops you paying for detailed drawings on a scheme the council was never going to approve, or one that costs far more than planned.
1. Which planning route applies?
- Permitted development: some work can go ahead without a planning application, within set limits.
- Lawful Development Certificate: optional confirmation that your permitted development project is lawful. See our guide to Lawful Development Certificates.
- Householder planning permission: needed where a project goes beyond permitted development.
Permitted development rights do not apply to flats and maisonettes. They can be restricted for listed buildings, in conservation areas, under Article 4 directions, or by earlier planning conditions. See permitted development vs planning permission.
2. Site constraints
Check for a conservation area or listed status. Check for Green Belt, where councils look at how much the house has grown compared with the original dwelling. Also check for flood zones (which may need a Flood Risk Assessment), Tree Preservation Orders, protected species (which may need an ecology survey) and National Landscape designation. None of these automatically rules a project out, but they shape what is realistic.
3. Neighbours and boundaries
Councils assess loss of light, overlooking and whether an extension would feel overbearing. Many use a 45-degree guideline next to a neighbour's window. If you are building on or near a shared boundary, the Party Wall etc. Act 1996 may require notice to adjoining owners.
4. Drains and sewers
If a public sewer runs within three metres of your build, you will usually need a Build Over Agreement from your water company. Check this at the start.
5. Structure
Knocking through walls or converting a loft usually means steel beams, new foundations or a strengthened floor. Calculations are produced by an independent qualified structural engineer.
6. Budget
Allow for design fees, structural calculations, Building Control, Party Wall costs, VAT, fit-out and contingency. See our house extension cost guide for St Albans and Hertfordshire.
7. Building Regulations
Whatever the planning route, the work will almost always need Building Regulations approval. See our guide to building regulations drawing packages.
Run a feasibility check on your project
Our free online Project Feasibility Check takes a few minutes and gives an initial view of your likely route and budget. To talk it through, book a free 30-minute consultation.
How Hampshire Build can help
Hampshire Build is a fixed-fee residential architectural design practice with more than 20 years' experience and a 95% planning approval rate. We specialise in extensions, loft conversions and garage conversions. Your measured survey is booked within 10 working days of your deposit. Your first drawings follow within 15 working days of the survey.
FAQs
Do I need a feasibility study for a small extension?
Even small extensions can be affected by conservation areas, sewers or neighbouring windows. A free check can save paying for drawings that won't be approved.
Can a feasibility study guarantee planning permission?
No. Only the council grants permission. A feasibility study reduces the risk by identifying issues early.
How long does a householder planning application take?
Councils usually have eight weeks to decide once an application is validated.
Do I still need Building Regulations if I don't need planning?
Almost always, yes. They are separate approvals.