Rear Extensions Guide for London Homeowners
Expanding your living space in London can be a smart alternative to moving house saving on costly Stamp Duty (SDLT), while also avoiding all the admin that comes with changing your property address.
Understanding Rear Extensions
In order to complete a rear extension, it starts with excavation (digging) to support the planned walls.
Once the foundations have set, the new walls will be built up. Once they’re at the desired hight, the roof will be installed.
Following the installation of the roof, doors and windows are installed making the structure weatherproof.
The new extension will then be completed with internal finishes, following which the extension will be complete.
The Rear Extension Process
The vast majority of rear extension progress through a series of key stages:
Step 1: Survey and Design
A detailed site survey is completed; this will assess what can be done from the perspective of size and height while also assessing the impact on those properties to its perimeter. Architects and engineers then create bespoke designs to fit the homeowner’s directions.
Step 2: Planning Permission
Once the finalised designs are agreed, these will be submitted to the local planning authority via am planning application. Upon receipt, the local authority planning officers will assesses the homeowner’s proposal against local regulations and policies. This process ensures developments are sustainable, and in keeping with the surrounding area. Approval times can vary from weeks to months depending on the project. Generally speaking, the more complex or aesthetically unique the proposed rear extension, the longer it will take for planning consideration.
Rear extensions can also be progressed via permitted development which negates the need to go through this step. We would advise discussing this point with an experienced architect.
Step 3: Party Wall Notices
Once the final approved design is ready, a party wall surveyor will be in a position to serve party wall notices on adjoining owners affected by the works.
These are required by law at least 1 month before work commences.
After receiving the notice, adjoining owners have three response options:
- Consent, allowing the work to proceed without further steps
- Dissent and appoint a surveyor to represent their interests
- Dissent and appoint an agreed surveyor (one surveyor acts for both owners)
Regardless of the adjoining owner’s response, the building owner is responsible for the surveyor costs.
In the event of dissent, one or two surveyors will:
- Review architectural and structural plans
- Conduct a schedule of condition to record the current state of the neighbouring property
- Agree a party wall award detailing the scope of work, protections, access arrangements, and procedures for handling any damage
In our experience, the protections afforded to adjoining owners by the Party Wall etc. Act 1996, are likely to result in the following principles coming into play:
Sectional Hit and Miss Bays
If the extension’s flank wall and the foundations that support that wall, are in close proximity to any neighbouring structures, it’s likely they will need to be dug in 1m sequential hit and miss bays. Excavating in this manner will reduce the risk of movement and consequential damage to those neighbouring structures. While this a
Is a common request for party wall surveyors, it’s also likely building control will require this approach. You can hear a little more about this type of construction on our video.
Eccentrically Loaded Foundation
If the extension’s flank wall and the foundations are built up to, or in very close proximity to the boundary line, the foundations will need to be eccentric type. This form of foundation avoids future neighbouring owner claims for trespass and damage in the form of the cost of cutting back and removing the trespass. You can hear a little more about this on our video.
Trial Pit Requirement
If the planned extension’s location is alongside an existing neighbouring structure or extension, as part of the completion of the structural design, you will need to dig a trial pit alongside the neighbouring structure or extension. This determines if the current foundations project onto your property and ensures accuracy in the foundation design. You can hear a little more about this on our video.
Garden Access
If the planned extension’s location adjoins the boundary line, as part of the process of construction, the contractors will require access onto the neighbouring land or garden to enable them to safely and neatly construct the proposed extension’s flank wall. You can hear a little more about this on our video.
Step 4: Rear Extension Works Start
Once the party wall procedures have concluded and all other statutory consents are in place, the contractors can start on site. As set out above, they will start with digging the foundations, laying the concrete, following which they will construct the walls, roof, weatherproof the new extension and complete the internal finishes.
Step 5: Inspections and Certification
Throughout construction, regular inspections ensure compliance with building regulations. Upon completion, a final certification confirms the project’s safety and quality.
Step 6: Post-work Schedule of Condition Reports
Once the work is complete, the surveyor(s) inspect the neighbouring property. If damage is identified, the building owner is responsible for repairs or compensation.
What will it cost?
Rear extension costs do vary based on the size, finish, complexity and location of the project.
Typical price ranges for construction and fit-out are likely to be:
Small extensions (around 20m²):
Likely to be in the region of £40,000
Medium-sized builds (30–40m²):
Likely to be in the region of £65,000 and £100,000
Large or high-spec projects (45–50m²+):
Likely to be in the region of £100,000 plus.
In our experience, houses that share party walls or are built in close proximity to other houses or structures tend to incur higher basement costs due to increased complexity.
Our Take
Rear extensions are an excellent way to increase the size of your home and change the way the living space works for you.
From the perspective of residential construction works, they’re actually quite straightforward. Careful planning can also mean you may not need to vacate the property during the works.
As with any planned construction work, they require meticulous planning, professional expertise, and adherence to legal and safety standards. While the costs of this form of construction are by no means low, they will increase the property value post-work.
Be sure to employ an architect, engineer, party wall surveyor, and contractor with specific experience in rear extensions.
Team Qualifications
Our team of surveyors are not only highly experienced but importantly they are also qualified.
We’re proud to confirm our surveyors hold membership status and accreditation to some of the world’s leading professional governing bodies including; the Royal Institute of Chartered Surveyors (RICS), the Chartered Institute of Arbitrators (CIArb), The Chartered Association of Building Engineers (CABE) and the Pyramus and Thisbe Club (P&T).
