November 6, 2026

House Extension Without Planning Permission: A Guide for Detached Homes in London

A House Extension can create a larger kitchen-diner, a brighter family room or space to work from home without moving. For a detached home in Greater London, a single-storey rear extension may be possible under permitted development rights rather than through a full planning application.


The starting limits are straightforward: up to 4 metres deep from the original rear wall and 4 metres high overall, with a 3-metre eaves limit when within 2 metres of a boundary. Larger rear extensions of up to 8 metres may be possible through the prior approval process.


However, those dimensions are not a complete permission checklist. Your property's planning restrictions, the position of its original walls and the design of the proposed work all need checking before construction begins.



Key Takeaways

  • For detached-house rear extensions, the starting limits are 4 metres deep and 4 metres high, with 3-metre eaves near boundaries.
  • Measure from the original house, not automatically from a later attached garage.
  • Qualifying larger rear extensions up to 8 metres require the prior approval process.
  • Building regulations remain essential, and an LDC can document planning lawfulness.



Table of Contents

Can Your House Extension Use Permitted Development Rights?


Permitted development rights allow certain home improvements without a full planning application, provided the work meets the applicable conditions and limits. They can cover extensions, loft conversions and outbuildings, but each type of work has its own requirements.


This guide focuses on single-storey rear extensions to detached houses in Greater London. Do not apply these dimensions automatically to other property types or locations.


Before designing your House Extension, check whether the property has restrictions that affect permitted development. Particular care is needed with:

  • Homes in conservation areas.
  • Properties in national parks or designated landscape areas.
  • Listed buildings.
  • Any property where permitted development rights may be restricted or removed.


Do not assume that a listed building or a home in a designated area can use the standard route. Ask the local planning authority or a planning professional to establish which permissions and consents your proposal needs.

Understand the Approval Routes Before Choosing a Design


There are three separate issues to resolve: whether the extension needs planning permission, whether it requires prior approval, and how it will comply with building regulations.


A House Extension that does not require a full planning application is therefore not a project without rules or paperwork.

How Far and How High Can You Extend?


For the detached-home rear extension considered here, use these three measurements as your initial design checks.


1. Standard rear extension depth: 4 metres

Measure the proposed projection from the original rear wall, not automatically from the furthest wall currently standing. An earlier addition can change the appearance of the house without changing the correct measurement baseline.


A depth of up to 4 metres is the standard starting limit for this type of House Extension. A deeper proposal may need the larger rear extension prior approval route.


2. Overall extension height: 4 metres

The overall height limit is 4 metres. A flat, mono-pitched or gabled roof can be considered, provided the design satisfies the relevant height requirements.


Check the roof dimensions early. Choosing a roof shape first and checking compliance afterwards can leave you redesigning the proposal.


3. Eaves height near a boundary: 3 metres

Where the extension is within 2 metres of a property boundary, the eaves must not exceed 3 metres.


Overall height and eaves height are different measurements. A roof that stays below the overall height limit can still fail the boundary-related eaves test.


These figures are useful screening checks, not proof that every aspect of a House Extension qualifies. Confirm the complete proposal against the applicable planning conditions.

Measure From the Original House, Not Just the Existing House


The “original house” means the house as it was first built, or as it stood on 1 July 1948 if it was built before that date. This distinction is particularly important where a garage or previous extension projects behind the main rear wall.


When an attached garage can affect the measurement

An attached garage that forms part of the original house can affect the rear-wall baseline. Its relationship to the original building needs to be established rather than assumed from how it looks today.


A garage added later does not become part of the original house simply because it is attached.


For example, consider an older house with an attached garage added in 1960. That later garage should not be used to claim additional extension depth from its rear wall. The relevant original rear wall remains the starting point.


Why the construction date needs context

For a house built after July 1948, the relevant baseline is the house as first built. Avoid treating every garage built after 1948 as a later addition without checking the age and original configuration of the house itself.


A separate, detached garage does not provide a rear-wall baseline for extending the house.


Before finalising your House Extension drawings, establish:

  • When the house was built.
  • Which walls belonged to its original footprint.
  • Whether an attached garage was original or added later.
  • Whether existing additions could lead to an incorrect depth measurement.

Can a Detached House Have an 8-Metre Rear Extension?


A qualifying detached house may be able to accommodate a single-storey rear extension extending up to 8 metres through the larger home extension prior approval process.


This is not the same as proceeding under the standard 4-metre limit without further planning steps.


How neighbour consultation works

The council notifies neighbours, who have 21 days to raise concerns. Where objections arise, the council considers the proposal's effect on neighbouring amenity, including daylight, privacy and the presence of the new building.


Concerns may lead to further scrutiny or a need to adjust the design. An objection is not something to ignore, but the process involves a council assessment rather than simply a neighbour deciding the outcome.


Do not treat a lack of neighbour objections as permission to start immediately. Confirm that the required council process has been completed before beginning work.


Discussing your House Extension with neighbours early can help identify concerns while the design is still flexible. Show the proposed depth, roof height and boundary relationship clearly.

Design the House Extension Around Everyday Use


Extra floor area is most useful when it solves a specific problem. A deeper room alone may not improve a disconnected ground-floor layout.


Start by deciding what the existing home lacks:

  • A kitchen with enough room for dining and family activity.
  • A better connection between the kitchen, dining room and garden.
  • A separate family space or work-from-home area.
  • A utility room that keeps laundry away from the main living area.
  • Practical storage or a ground-floor shower room.


Balance open-plan space with functional rooms

Removing an internal kitchen wall can help connect old and new spaces. A partition can then create a utility room without giving up the main open-plan layout.


Storage, washing facilities and quieter areas deserve attention alongside the kitchen-diner. A useful House Extension should improve how the entire ground floor works, not just add a large room at the back.


Use glazing to strengthen the garden connection

Large bifold doors and a wide rear window can bring more natural light into the extended living area and create a stronger connection to the garden.


Coordinate these openings with the furniture layout, kitchen arrangement and the building regulations requirements for the project.


Choose the roof and materials together

Flat roofs are generally simpler to construct and can cost less, while pitched roofs may sit more comfortably alongside traditional London homes.


Brick, render and cladding should complement the existing property. Consider durability, maintenance and the home's architectural character as well as the initial appearance.

House Extension Costs and Value: What the Examples Show


Project examples help illustrate the relationship between size, layout and cost, but they are not fixed price guides. The figures below relate to particular schemes and should not be treated as current quotations for another property.


A 6.6-metre-wide, 3-metre-deep extension

One example used a rear extension measuring 6.6 metres wide by 3 metres deep, creating approximately 19.8 square metres of additional footprint.


The wider ground-floor redesign connected the kitchen and dining areas, introduced bifold doors and a rear window, and used a partition to provide utility space. The overall remodel also included additional living space, storage and washing facilities.

  • Reported 2025 cost: approximately £66,000 to £90,000.
  • Reported value increase: approximately £70,000.
  • Design lesson: combine new floor area with improvements to the existing layout.


The reported value gain does not establish a guaranteed profit. It falls within the stated cost range, and another House Extension may produce a different financial result.


A 4-metre extension with insulation improvements

Another example combined a 4-metre extension with a lounge, shower room and lean-to roof. The reported construction cost was approximately £2,400 per square metre.


Upgraded insulation was associated with a reported 18% improvement in energy efficiency for that project. This is a project-specific result, not an expected outcome for every extension.


Will an extension increase your home's value?

An indicative uplift of around 6.5% would equate to approximately £36,000 on a property worth £556,000. That calculation illustrates a possible value change, not a valuation of your own home.


Assess a House Extension against both its cost and its practical benefit. A better kitchen, useful storage and space for a growing family can matter even when the resale uplift does not cover the entire build cost.

Why Building Regulations and an LDC Still Matter


Permitted development does not remove the need to comply with building regulations. Planning lawfulness and technical construction standards are separate matters.


The design and construction need to address:

  • Structural integrity.
  • Insulation and energy efficiency.
  • Electrical installations.
  • Drainage.
  • Ventilation.
  • Fire safety.


A Lawful Development Certificate, or LDC, serves a different purpose. It provides formal confirmation that the proposal is lawful in planning terms.


Obtaining an LDC for a permitted development House Extension can reduce uncertainty and provide useful documentation when selling the property. It does not replace building regulations compliance or make a non-compliant design acceptable.

Common Mistakes to Avoid Before Building


  1. Measuring from a later garage: establish the original house footprint before calculating extension depth.
  2. Checking only the roof's highest point: verify the eaves height where the proposal is close to a boundary.
  3. Assuming every detached house qualifies: investigate local restrictions and designated status first.
  4. Treating an 8-metre extension like a standard proposal: allow for the required prior approval process.
  5. Confusing an LDC with technical approval: address planning lawfulness and building regulations separately.
  6. Budgeting from a value-uplift headline: use project-specific costs and avoid assuming a guaranteed return.

Your Pre-Design Checklist


Before committing to a House Extension, work through these steps:


  1. Confirm the property's type, location and planning restrictions.
  2. Identify the original rear wall and any later additions.
  3. Set out the proposed depth, overall height and eaves height.
  4. Check the distance to property boundaries.
  5. Establish whether standard permitted development, prior approval or a planning application is appropriate.
  6. Plan the layout around the rooms and functions you actually need.
  7. Include building regulations requirements in the design.
  8. Seek formal confirmation of planning lawfulness where appropriate.
  9. Prepare a budget for the defined project rather than relying on another home's results.


The best starting point is not the largest possible footprint. It is a compliant, well-planned extension that improves the way your home works.

Frequently Asked Questions

Can I build a House Extension without planning permission in London?

A qualifying detached house may use permitted development rights for a single-storey rear extension. The proposal must meet the applicable dimensions and conditions, and you must check whether property-specific restrictions affect those rights.

How far can I extend a detached house at the rear?

The standard depth discussed here is up to 4 metres from the original rear wall. A qualifying larger extension exceeding 4 metres and extending up to 8 metres may be possible through prior approval.

Can I measure extension depth from my garage?

Only if the attached garage forms part of the relevant original house footprint. A later garage addition does not reset the measurement baseline, and a detached garage does not provide a rear wall for extending the house.

Does permitted development mean I can ignore building regulations?

No. Structural safety, insulation, electrical work, drainage, ventilation and fire safety still need to meet the relevant building regulations requirements.

Is a Lawful Development Certificate the same as planning permission?

No. An LDC confirms planning lawfulness, such as whether a proposal qualifies under permitted development. It does not replace building regulations compliance or grant permission for a proposal that fails the relevant conditions.

Will a House Extension pay for itself?

Not necessarily. An extension may increase the property's value, but the uplift is not guaranteed to exceed construction costs. Consider the financial outcome alongside the improvement to everyday living.

PREVIOUSNEXT
Top Trends in Commercial Architecture: What to Expect in 2023

Join our Newsletter

Subscribe to our monthly newsletter and get updates and industrial insights delivered to your inbox.

Keep Me Updated

Back to top