Getting planning permission for a home extension in London can feel overwhelming. Many applications are delayed or refused due to avoidable mistakes — often caused by unclear drawings, missing context or policy conflicts.
In this guide, we explain the most common planning permission mistakes and how to improve your chances of approval in 2026.
Why Do Planning Applications Get Refused?
The most common reasons planning applications for home extensions are refused include:
- – Missing contextual drawings
- – Ignoring local council policies
- – Overlooking neighbouring properties
- – Inconsistent drawing scales
- – Incomplete documentation
- – Poor design in conservation areas
- – Failure to address sustainability requirements
Planning officers must clearly understand what you are proposing and how it affects neighbours and the surrounding area. If anything is unclear, your application is at risk.
Common Planning Permission Mistakes Homeowners Make
1. Missing Detail or Context
Planning officers need accurate dimensions, scale, materials and site context. Drawings should show:
- – Existing and proposed floor plans
- – Elevations
- – Sections
- – Relationship with neighbouring properties
- – Site boundaries and orientation
Without this information, officers cannot properly assess impact.
2. Ignoring Local Planning Policies
Every London borough has specific planning policies covering:
- – Extension depth and height
- – Roof alterations
- – Design in conservation areas
- – Overlooking and daylight impact
- – Materials and appearance
Applications that fail to align with local policy are frequently refused — even if the design appears reasonable.
Before submitting, always review your borough’s Local Plan and Supplementary Planning Documents (SPDs).
3. Inconsistent Drawing Scales
Using multiple scales across drawings creates confusion and slows down assessment. Standard practice is:
- – 1:100 for plans and elevations
- – 1:50 for detailed drawings
- – 1:1250 for location plans
Consistency improves clarity and professionalism.
4. Incomplete Drawing Sets
Planning authorities typically require:
- – Location plan
- – Block plan
- – Existing and proposed floor plans
- – Existing and proposed elevations
- – Sections (where necessary)
- – Design and Access Statement (if required)
Missing documents can lead to validation delays or refusal.
5. Overlooking Site Constraints
Small developers and homeowners often overlook site-specific constraints such as:
- – Conservation areas
- – Flood zones
- – Tree Preservation Orders
- – Party Wall implications
- – Access restrictions
Each constraint can significantly influence approval.
How to Improve Your Chances of Planning Approval
✔ Provide Clear 2D and 3D Drawings
Visual clarity makes a major difference. Alongside technical plans, 3D visualisations help planning officers and neighbours understand:
- – Scale
- – Massing
- – Design impact
- – Relationship to surrounding properties
This reduces objections and misunderstandings.
✔ Show the Wider Context
Applications should demonstrate how the extension fits into the streetscape. Including neighbouring buildings helps officers assess:
- – Overlooking
- – Overshadowing
- – Character impact
Context reduces uncertainty.
✔ Learn from Previous Planning Decisions
Reviewing previous approvals and refusals in your area can reveal:
- – What case officers accept
- – Common design concerns
- – Policy interpretation trends
Planning is partly technical and partly strategic.
✔ Stay Up to Date with Policy
National and local planning policies evolve. Sustainability requirements, energy efficiency standards and biodiversity expectations are increasing in 2026.
Ensuring compliance from the outset avoids costly redesigns later.
Why Work with Studio Casa?
Planning permission is not just about drawings, it’s about strategy, clarity and understanding how councils assess proposals.
At Studio Casa, we combine architectural design, structural thinking and policy experience to prepare applications that are clear, compliant and well presented.
We support homeowners by:
- – Detailed 2D technical drawings
- – Professional 3D visualisations to clarify scale and massing
- – Reviewing local planning policies and recent decisions
- – Identifying risks before submission
- – Advising on the most appropriate application route
Our goal is simple: reduce uncertainty, minimise delays and give your home extension the strongest possible chance of approval.
Before submitting a full planning application, many clients start with a Planning Feasibility Assessment to evaluate risk and strategy.
👉 Explore our Case Studies to see real approved projects across London.
👉 Read our Extension Costs London 2026 Guide to plan your budget.
Planning an Extension in London?
If you’re planning a home extension, loft conversion or new build, careful preparation can make the difference between approval and refusal.
Book a Planning Feasibility Consultation and reduce risk before submitting your application.