Why a Planning Approval Does Not Mean Your Extension Is Ready to Build
Receiving planning permission for a house extension is a significant step, but it is not the end of the design process.
Planning drawings are mainly prepared to demonstrate the size, position, appearance and overall impact of a proposal. They usually do not contain enough technical information for a builder to price or construct the project accurately.
Before work begins, the design normally needs to be developed into a coordinated package of Building Regulations and construction information.
What do planning drawings usually show?
A typical planning submission may include:
- existing and proposed floor plans;
- existing and proposed elevations;
- a site location plan;
- a block or site plan;
- roof plans where relevant;
- basic dimensions and material notes.
These drawings allow the local planning authority to understand what the finished development will look like and how it relates to the existing house and neighbouring properties.
However, planning drawings do not usually explain exactly how every part of the extension will be built.
What is missing from planning drawings?
A builder may need considerably more information before starting construction.
This can include:
- foundation sizes and construction;
- floor build-ups;
- wall construction;
- roof structure and insulation;
- steel beam positions;
- structural openings;
- cavity barriers and fire protection;
- ventilation requirements;
- drainage layouts;
- thermal calculations;
- window and door performance;
- junction details;
- damp-proofing;
- electrical and safety requirements.
Without this information, important decisions may be left until work is already underway.
That can lead to delays, unexpected costs and disagreements over what was included in the builder’s quotation.
What are Building Regulations drawings?
Building Regulations drawings explain how the proposed extension is intended to meet the relevant technical standards.
They are normally more detailed than planning drawings and may include:
- construction notes;
- wall, floor and roof specifications;
- insulation thicknesses;
- fire-safety measures;
- ventilation requirements;
- drainage information;
- structural design references;
- detailed sections through the building;
- notes covering accessibility and safety.
The purpose is not simply to obtain approval. A good technical package should also provide the builder with a clearer basis for pricing and construction.
Will a structural engineer be required?
Many extensions require input from a structural engineer.
This is particularly common where the project includes:
- removal of load-bearing walls;
- large openings for sliding or bi-fold doors;
- steel beams;
- complex roof alterations;
- unusual ground conditions;
- retaining walls;
- substantial areas of glazing.
The architect and structural engineer should coordinate their information so the architectural drawings reflect the structural design.
Leaving this until construction has started can result in changes to ceiling levels, room layouts or the size of structural elements.
Why early coordination matters
A planning design may look simple on paper, but extensions often involve several overlapping technical issues.
For example, the position of a steel beam can affect:
- ceiling heights;
- roof construction;
- insulation;
- lighting;
- ventilation ducts;
- door openings;
- kitchen layouts.
Similarly, drainage routes may affect foundation design, floor levels and the position of external walls.
Resolving these issues before construction begins is usually far easier and less expensive than trying to solve them on site.
Can a builder price from planning drawings?
A builder can sometimes provide an initial estimate from planning drawings, but the price may contain significant assumptions.
Different builders may interpret the same limited information in different ways. One quotation may include certain insulation, drainage or structural works while another may not.
A more detailed drawing package helps quotations to be compared on a more consistent basis.
It also reduces the risk of additional costs appearing later because something was not clearly shown or specified.
What should be completed before work starts?
The exact requirements will depend on the project, but a typical extension may need:
- Planning permission or confirmation that the work is permitted development.
- Building Regulations drawings and submission.
- Structural calculations.
- Drainage and foundation coordination.
- Party wall advice where applicable.
- Any planning conditions discharged.
- A detailed builder’s quotation.
- A clear agreement covering the scope of work.
Beginning construction without these matters being properly considered can create avoidable uncertainty.
What happens if the technical design changes the approved scheme?
Technical development sometimes identifies issues that require minor changes to the planning design.
For example, the roof depth may increase because of insulation, or a structural solution may affect an opening or elevation.
Minor changes may be acceptable, but more significant alterations could require a formal amendment or a new planning submission.
This is another reason to develop the technical design before materials are ordered or construction begins.
The value of a coordinated drawing package
A well-prepared technical package provides benefits for the homeowner, builder and Building Control inspector.
It can help to:
- obtain more reliable quotations;
- reduce assumptions;
- identify technical conflicts early;
- improve communication;
- reduce delays;
- limit unexpected additional costs;
- provide a clearer record of the agreed design.
Planning approval confirms that the principle and appearance of the development are acceptable. Detailed technical design is what turns that approval into a buildable project.
Conclusion
Planning permission should be viewed as one stage within a wider design and construction process.
Before starting an extension, homeowners should make sure the planning design has been developed into coordinated Building Regulations and construction information.
Investing in this work before construction begins can help protect the design, control costs and reduce problems on site.