What Counts as the “Original House” in Permitted Development?
One of the most common reasons permitted development projects fail is a misunderstanding of what the planning system means by the “original house”.
Homeowners often 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 where extensions were built decades ago or by previous owners.
This guide explains what counts as the original house, how councils assess it, and why getting this wrong can invalidate an otherwise compliant project.
What Is the “Original House”?
In planning terms, the original house is defined as the house as it stood on 1 July 1948, or as first built if constructed after that date.
This definition is set out in the General Permitted Development Order (GPDO) and applies across England.
Any part of the building added after this date — including:
Rear extensions
Side extensions
Loft conversions
Outbuildings physically attached to the house
is not considered part of the original house for permitted development calculations.
Why the Original House Matters
Permitted development limits for extensions, lofts and outbuildings are all measured from the original house.
This affects:
Maximum projection depths
Overall footprint coverage
Height and eaves calculations
Roof volume allowances
Whether extensions encroach on side elevations
Even small errors can result in a scheme requiring planning permission.
Previous Extensions Still Count — Even If You Didn’t Build Them
A very common misconception is that extensions built by previous owners “don’t count”.
They do.
For permitted development purposes:
All post-1948 extensions are included in PD calculations
PD allowances are not reset when a property is sold
Lawful but unauthorised extensions still affect PD rights
This frequently causes problems where multiple modest extensions have been added over time.
Building Adjacent to an Original Side-Facing Wall
One of the most misunderstood permitted development rules applies where works are proposed adjacent to an original side-facing wall, even if that wall also forms part of the rear elevation.
This commonly affects:
End-of-terrace houses
Corner plots
Properties with staggered rear elevations
Under permitted development rules, where a rear extension is built adjacent to an original side-facing wall, it must:
Be single storey, and
Not exceed half the width of the original house
If these limits are exceeded, the proposal will not qualify as permitted development and will require planning permission or a revised design.
This issue is one of the most common reasons Lawful Development Certificates are refused, despite homeowners believing their extension is “entirely to the rear”.
Extending From an Existing Extension
Another frequent error is assuming you can extend further from an existing extension under permitted development.
Because PD measurements are taken from the original house, this means:
Previous extensions reduce the remaining PD allowance
Adding to an existing extension may exceed projection or footprint limits
Seemingly minor additions can invalidate PD compliance
This is particularly problematic where historic extensions were never properly documented.
How Councils Determine the Original House
Local authorities typically assess the original house using:
Historic planning records
Old Ordnance Survey maps
Aerial imagery
Building Control records
Planning histories from neighbouring properties
Where evidence is unclear, councils usually take a conservative approach, placing the burden of proof on the homeowner.
Do You Need a Lawful Development Certificate?
If your proposal relies on permitted development — especially where:
The original house is unclear
Previous extensions exist
A side-facing wall is involved
a Lawful Development Certificate (LDC) is strongly recommended.
An LDC provides written confirmation that the works are lawful and is often required by solicitors when selling or refinancing a property.
How Draw and Plan Helps
At Draw and Plan, we:
Assess your original house footprint before any drawings are produced
Identify PD constraints early to avoid redesign costs
Prepare compliant Lawful Development Certificate drawings
Act as your agent with the local authority
Design fallback options where PD rights are marginal
Our approach is evidence-based, practical, and focused on certainty.
Thinking of Extending Your Home?
Before committing to builders or designs, it’s essential to confirm what legally counts as your original house.
Contact Draw and Plan for a permitted development assessment and compliant drawings.
