One of the most common reasons Permitted Development (PD) projects fail is a misunderstanding of what planning legislation means by the “original house.”
Many homeowners assume measurements are taken from the house as it exists today. In reality, Permitted Development Rights are almost always assessed against the original form of the house—even if extensions were built decades ago or by previous owners.
Understanding this definition is essential before designing an extension, loft conversion or outbuilding.
What Is the “Original House”?
For Permitted Development purposes, the original house is defined as:
- The house as it stood on 1 July 1948, or
- If built after that date, the house as originally constructed.
This definition comes from the Town and Country Planning (General Permitted Development) (England) Order 2015 (GPDO) and applies throughout England.
Anything added after that date is not considered part of the original house, including:
- Rear extensions
- Side extensions
- Loft conversions
- Conservatories
- Attached garages and outbuildings
These additions may be lawful, but they still affect how much Permitted Development allowance remains.
Why the Original House Matters
Almost every Permitted Development calculation is measured from the original house. This affects:
- Maximum extension depth
- Overall site coverage
- Roof enlargement volume
- Eaves and ridge height calculations
- Whether an extension projects beyond an original side wall
Using today’s footprint instead of the original house can easily result in a proposal requiring planning permission.
Previous Extensions Still Count
One of the biggest misconceptions is that extensions built by previous owners somehow “don’t count.” Unfortunately, they do.
For Permitted Development purposes:
- All post‑1948 extensions count towards PD allowances.
- Permitted Development Rights do not reset when ownership changes.
- Historic extensions continue to reduce the remaining allowance, even decades later.
This is particularly common where several modest extensions have been added over many years.
Building Next to an Original Side‑Facing Wall
A frequently misunderstood rule concerns development adjacent to an original side‑facing wall, even where that wall also forms part of the rear elevation. This commonly affects:
- End‑of‑terrace houses
- Corner plots
- Houses with staggered rear elevations
- Properties with projecting rear wings
Where a rear extension is built alongside an original side‑facing wall, it must normally:
- Be single storey, and
- Not exceed half the width of the original house.
If these limitations are exceeded, the proposal will generally fall outside Permitted Development and require planning permission. This is one of the most common reasons Lawful Development Certificate applications are refused.
Extending an Existing Extension
Another common mistake is assuming that you can simply build further from an existing extension. Because all measurements are taken from the original house:
- Existing extensions reduce your remaining PD allowance.
- Additional projection may exceed the permitted limits.
- Site coverage restrictions may already have been reached.
Even relatively small additions can result in a proposal no longer qualifying as Permitted Development.
How Councils Identify the Original House
Where there is uncertainty, local planning authorities will usually examine historical evidence, including:
- Historic planning permissions
- Building Control records
- Ordnance Survey maps
- Historic aerial photography
- Neighbouring planning histories
- Archived property records
Where evidence is inconclusive, the burden of demonstrating the original house normally rests with the applicant.
Why a Lawful Development Certificate Is Recommended
If your proposal relies on Permitted Development Rights—particularly where:
- The original footprint is unclear
- Historic extensions exist
- An original side‑facing wall is involved
- Previous alterations have taken place
…obtaining a Lawful Development Certificate (LDC) is strongly recommended. An LDC provides written confirmation from the local planning authority that the proposed works are lawful. It can also:
- Provide certainty before construction begins
- Help avoid neighbour disputes
- Assist when selling or remortgaging your property
- Reduce the risk of future enforcement action
Although not legally required, an LDC is often well worth obtaining.
Common Mistakes to Avoid
Many Permitted Development applications fail because homeowners:
- Measure from the existing house instead of the original house
- Assume previous extensions don’t count
- Ignore historic planning conditions
- Forget that Article 4 Directions may remove PD Rights
- Miscalculate roof enlargement volumes
- Build alongside an original side wall without checking width restrictions
Checking these issues before any drawings are prepared can save significant time and expense.
Why Professional Advice Matters
Permitted Development legislation is far more detailed than many homeowners realise. A proposal that appears fully compliant can fall outside PD because of a historic extension built decades earlier or a little‑known planning condition. Establishing the original house should always be one of the first stages of any feasibility assessment.
Need Help Assessing Your Permitted Development Rights?
At Draw & Plan, we establish the footprint of your original house before any drawings are prepared, helping to identify Permitted Development constraints at the earliest stage.
Our Planning Drawings service includes advice on Permitted Development Rights, Prior Approval and Lawful Development Certificate applications, helping homeowners avoid costly redesigns and enforcement risks.
If you’re new to Permitted Development, our guide Permitted Development Rights Explained: What Can You Build Without Planning Permission? provides an overview of the rules and common pitfalls.
Once your proposal has been confirmed as lawful, our Building Regulations Drawings service provides the detailed technical drawings required before construction can begin.
Ready to discuss your project? Contact us today for a free, no‑obligation quotation.
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Whether a property qualifies for Permitted Development depends on its planning history, previous extensions and local planning restrictions. Always seek advice from a suitably qualified professional or your local planning authority before commencing work.