What is a House Extension?
A house extension is the most transformative renovation project a London homeowner can undertake. Whether you need a larger kitchen, an open-plan family living space, a ground-floor bedroom, or a home office, extending your home allows you to gain the space you need without the upheaval and expense of moving.
London's housing stock — predominantly Victorian terraces, Edwardian semis, and post-war detached houses — lends itself well to extension. The narrow side returns of terraced houses are begging to be filled in, the shallow rear additions of Edwardian properties can be replaced with full-width extensions, and detached houses in outer boroughs often have generous plots with scope for significant expansion.
This guide covers every aspect of extending your home in London in 2025. From the different types of extension and their suitability for various property styles, through planning permission and Building Regulations, to costs, timelines, and the design process — everything you need to make informed decisions about your project.
Types of House Extensions
Understanding the different types of house extensions available is essential for making the right choice for your property, budget, and requirements. Each type has distinct advantages, cost implications, and suitability for different property types.
Rear Extension
Side Return Extension
Wraparound Extension
Double Storey Extension
Conservatory
Orangery
Garden Room / Outbuilding
Planning Permission in London
Planning permission for house extensions in London depends on the type, size, and location of the extension, as well as the status of your property and its surroundings.
Permitted Development Rights for Extensions
Single-storey rear extensions can often be built under permitted development rights. The key limits are:
- Detached houses: extend up to 8 metres from the rear wall (or 4 metres under standard PD, with prior approval for 4–8m)
- Semi-detached and terraced houses: extend up to 6 metres (or 3 metres standard, with prior approval for 3–6m)
- Maximum eaves height of 3 metres
- Maximum overall height of 4 metres
- Extension must not cover more than half the area of land around the original house
- Materials must be similar in appearance to the existing dwelling
When You Need Planning Permission
You will need to apply for planning permission if:
- The extension exceeds PD size limits
- You are building a two-storey extension (limited PD rights exist but are restrictive)
- Your property is in a conservation area (side extensions and any extensions visible from the highway typically need permission)
- Your property is a listed building (Listed Building Consent required for all changes)
- Your PD rights have been removed by an Article 4 direction or a condition on the original planning permission
- The extension is to the front or side elevation in certain circumstances
The Larger Home Extension Scheme (Prior Approval)
The government's "larger home extension" scheme allows single-storey rear extensions up to 6m (semi/terrace) or 8m (detached) under a prior approval process. This requires notifying the council, who consult adjoining neighbours. If no objections are received within 21 days, approval is granted. If objections arise, the council makes a determination based on the impact on amenity. This scheme has been made permanent and is widely used across London.
Conservation Areas
In conservation areas across London — from Hampstead Village to Dulwich, Blackheath to Richmond — PD rights for extensions are significantly curtailed. Side extensions typically require planning permission, and even rear extensions may need approval if they affect the character of the conservation area. Design quality, materials, and proportions are scrutinised much more closely.
Building Regulations
All house extensions require Building Regulations approval. The regulations ensure the extension is structurally sound, energy efficient, fire safe, and properly drained.
Part A — Structure
Foundations must be designed to suit the ground conditions. In London, this often means strip foundations at least 1 metre deep to avoid tree root damage (London clay is highly susceptible to subsidence from trees). Near large trees, depths of 2–3 metres or piled foundations may be required. A structural engineer must design the foundation, steel beams, and any openings between the existing house and the new extension.
Part B — Fire Safety
Extensions within 1 metre of a boundary must have fire-resistant external walls (minimum 1-hour fire resistance). Windows and doors in walls close to boundaries may need to be fire-rated or restricted in size. If the extension creates an inner room (a room accessed only through another room), additional fire escape provisions may be required.
Part L — Conservation of Energy
This is often the most impactful regulation for extensions. New walls must achieve a U-value of 0.28 W/m²K, roofs 0.18 W/m²K, and floors 0.22 W/m²K. Windows and doors must be double-glazed as a minimum (U-value 1.6 W/m²K or better). Since the 2022 updates to Part L, extensions must also demonstrate compliance with a simplified SAP assessment, pushing designs towards better insulation, efficient heating, and potentially renewable energy sources.
Part H — Drainage
The extension will cover existing ground, so surface water drainage must be managed. In most London boroughs, Sustainable Drainage Systems (SuDS) are required — meaning rainwater cannot simply be discharged into the combined sewer. Soakaways, permeable paving, or rainwater harvesting are common solutions. Foul drainage from any new kitchen or bathroom must connect to the existing drainage system with appropriate falls and access points.
Part P — Electrical Safety
All electrical installations in the extension must comply with BS 7671 and be installed by a Part P registered electrician or inspected by Building Control.
House Extensions Costs in London 2025
Extension costs in London vary enormously depending on size, specification, structural complexity, and location. Here are realistic 2025 figures:
Base Construction Costs
| Type | Cost per sqm | Typical Total |
|---|---|---|
| Single-storey rear | £2,500–£3,800 | £50,000–£120,000 |
| Side return infill | £3,000–£4,200 | £35,000–£75,000 |
| Wraparound | £2,800–£4,500 | £80,000–£180,000 |
| Double storey | £2,500–£4,000 | £90,000–£200,000 |
| Orangery | £3,500–£5,500 | £60,000–£150,000 |
| Garden room | £2,000–£3,500 | £25,000–£80,000 |
What Affects the Cost
- Ground conditions: London clay near trees can require deep foundations or piling, adding £5,000–£20,000
- Glazing specification: Large structural glass panels, bifold doors, and roof lanterns can cost £15,000–£40,000
- Kitchen fit-out: If the extension houses a new kitchen, the kitchen itself adds £15,000–£60,000+
- Underfloor heating: A common addition, costing £3,000–£8,000 for a typical extension
- Drainage diversions: Moving existing drains that run beneath the extension footprint costs £3,000–£12,000
- Structural glazing: Frameless glass or minimal frame systems cost 2–3x standard bifold doors
Hidden Costs
- Thames Water build-over agreement: If the extension is built over a public sewer (common in London), you need Thames Water approval — £400 application fee plus any required modifications
- Party Wall Act: £1,500–£3,500 per neighbour for surveyor fees
- Landscaping reinstatement: Repairing garden damage from construction access — £2,000–£10,000
- Professional fees: Architect, structural engineer, and planning consultant — typically 10–15% of build cost
- Building Control fees: £800–£2,000
Quick Cost Summary
Timeline: How Long Does It Take?
House extensions are more complex than loft conversions due to groundworks, foundations, and greater planning requirements. Here is a realistic London timeline:
Design Phase (4–8 weeks)
Initial consultation, measured survey, concept designs, and design development. For larger extensions or those in conservation areas, this phase may include pre-application discussions with the local authority. We typically present two or three design options before developing the preferred scheme in detail.
Planning Phase (8–12 weeks)
A standard householder planning application in London takes 8 weeks for a decision, though many boroughs are running behind this target. Prior approval applications (larger home extension scheme) take 42 days. In conservation areas or for more complex schemes, allow 12 weeks. Pre-application advice, where offered, takes 4–6 weeks and can significantly smooth the formal application process.
Technical Design and Tender (4–6 weeks)
Detailed construction drawings, structural engineering calculations, Building Regulations submission, and contractor tendering. We recommend obtaining at least three competitive tenders from vetted contractors.
Construction Phase (12–20 weeks)
A typical single-storey rear extension takes 12–16 weeks to build. Wraparound and double-storey extensions take 16–20 weeks. The sequence is: site setup and demolition (1 week), foundations and groundworks (2–3 weeks), superstructure (walls, steelwork, roof — 3–4 weeks), weathertight enclosure (1–2 weeks), first fix services (2–3 weeks), plastering and screeding (1–2 weeks), second fix and finishing (3–4 weeks).
Snagging and Handover (2–3 weeks)
Final inspections, snagging, Building Control completion certificate, and handover documentation.
Timeline Summary
- Design4–8 weeks
- Planning8–12 weeks
- Construction12–20 weeks
- Finishing3–5 weeks
- Total7–12 months
The Design Process
At Hampstead Renovations, we follow a structured design process for every house extensions project. This process has been refined over hundreds of projects across North London and ensures that nothing is overlooked, budgets are managed, and the final result exceeds expectations.
1. Initial Brief & Site Visit
Every project begins with a conversation. We visit your property, listen to your requirements, understand your budget, and assess the feasibility of your ideas. For house extensions, this initial visit is crucial — we need to understand the existing structure, identify constraints, and discuss the range of options available to you. This meeting is free and without obligation.
2. Concept Design
Based on the brief, we develop two or three concept design options. These are presented as floor plans, sections, and 3D visualisations so you can understand how the space will look and feel. We discuss the pros and cons of each option, the cost implications, and any planning considerations. This phase typically takes 2–3 weeks.
3. Developed Design
Once you have chosen a preferred concept, we develop it in detail. This includes finalising the layout, specifying materials and finishes, developing the structural strategy with our engineer, and resolving all the technical details that affect how the space works. We provide a detailed cost estimate at this stage so you can make informed decisions about specification.
4. Planning Application (if required)
If planning permission is needed, we prepare and submit the application, including all supporting documents (design and access statement, heritage impact assessment for listed buildings, structural methodology for basements). We manage the application process, respond to any council queries, and negotiate with planning officers where necessary.
5. Technical Design & Building Regulations
We produce detailed construction drawings and specifications — the documents your contractor will build from. These include architectural plans, sections and elevations, structural engineering drawings, services layouts, and a comprehensive specification of materials and workmanship. We submit for Building Regulations approval and manage the approval process.
6. Tender & Contractor Appointment
We invite three to four vetted contractors to price the project from our detailed drawings and specification. We analyse the tenders, interview the contractors, and recommend the best appointment based on price, programme, experience, and references. We help you negotiate the contract terms and agree a realistic programme.
7. Construction & Contract Administration
During construction, we carry out regular site inspections to ensure the work complies with the design, specification, and Building Regulations. We chair progress meetings, manage variations, certify interim payments, and resolve any issues that arise. Our role is to protect your interests and ensure the project is delivered to the agreed quality, programme, and budget.
8. Completion & Handover
At practical completion, we carry out a thorough snagging inspection and produce a defects list for the contractor to address. We manage the Building Control final inspection, obtain the completion certificate, and compile a comprehensive handover pack including all warranties, certificates, maintenance guides, and as-built drawings.
Common Mistakes to Avoid
Over hundreds of house extensions projects across London, we have seen the same mistakes repeated. Learning from others' errors can save you thousands of pounds and months of frustration.
1. Not considering the impact on natural light
A common mistake is extending so far that the existing rooms behind the extension lose natural light. Deep single-storey extensions should incorporate roof lanterns, skylights, or clerestory windows to bring light into the middle of the plan. The "25-degree rule" is a useful guide: if a line drawn at 25 degrees from the centre of the nearest habitable room window hits the extension, light loss will be significant.
2. Underspecifying foundations near trees
London's clay soil is highly reactive — it shrinks in summer when trees extract moisture and swells in winter. Foundations near trees must be designed to NHBC standards, which can mean depths of 2–3 metres or more. Cutting costs by reducing foundation depth near trees is a recipe for subsidence and structural cracking.
3. Forgetting about drainage before building
Many London properties have drains running directly beneath where an extension will sit. Discovering this during construction is expensive and disruptive. A CCTV drain survey before design begins costs a few hundred pounds and can save thousands in redesign and drain diversion costs.
4. Choosing bifold doors without considering practicality
Bifold doors are the default choice for rear extensions, but they are not always the best. They stack to one side (blocking a corner), require a flat threshold (which needs careful detailing for weather tightness), and can feel draughty in winter. Sliding doors often provide a cleaner look, better thermal performance, and more usable wall space.
5. Not thinking about storage
Open-plan extension kitchens look beautiful in photographs but can lack storage in reality. Deep pan drawers, tall larder units, utility rooms, and hidden storage must be designed into the scheme from the start. An architect who understands kitchen ergonomics is essential.
6. Ignoring the relationship between old and new
The junction between the existing house and the new extension is architecturally critical. A flat ceiling that aligns with the existing ceiling height, consistent floor levels (no steps), and a considered material palette that either matches or deliberately contrasts with the original house — these details separate good extensions from mediocre ones.
How to Choose a Contractor
The choice of contractor is one of the most important decisions you will make in any renovation project. A good contractor delivers quality work on time and on budget; a poor one can cause delays, cost overruns, defective work, and enormous stress. Here is how to find and evaluate the right contractor for your project.
What to Look For
- Relevant experience: Ask to see completed projects similar to yours in type, scale, and specification. A contractor who specialises in basement conversions may not be the best choice for a period restoration, and vice versa. Request references from recent clients and, if possible, visit a completed project
- Insurance: Verify public liability insurance (minimum £5 million), employer's liability insurance (a legal requirement if they employ anyone), and professional indemnity insurance if they are providing any design input. Ask to see current certificates, not expired ones
- Trade body membership: Membership of the Federation of Master Builders (FMB), TrustMark, or the National Federation of Builders (NFB) provides some assurance of competence and financial stability. For specialist work, look for relevant accreditations (e.g., PCA for waterproofing, NICEIC for electrical)
- Financial stability: A contractor who goes bust mid-project is every homeowner's nightmare. Check Companies House for financial health, look for a stable trading history, and consider whether the company has sufficient resources to manage your project alongside their other commitments
- Communication style: During the quoting process, assess how responsive, clear, and professional the contractor is. This is a preview of how they will communicate during the project. If they are slow to return calls or vague in their quotes at this stage, it will not improve once they have your money
Red Flags to Avoid
- Quoting without visiting the site or seeing detailed drawings
- Requesting large upfront payments (more than 10–15% of the contract value)
- No written contract or a vague, one-page quotation
- Pressure to commit quickly or "special" discounts that expire
- Unable or unwilling to provide references from recent projects
- No insurance certificates available for inspection
- The quote is significantly lower than all others — this usually means something has been missed, not that they are offering better value
Questions to Ask
- How many similar projects have you completed in the last two years?
- Who will be the site manager/foreman for my project, and how many other projects will they be managing simultaneously?
- What is your proposed programme (start date, key milestones, completion date)?
- How do you handle variations and additional work — what is your day rate for unforeseen items?
- What warranty do you provide on your work?
- Can I speak to three recent clients whose projects are similar to mine?
Case Studies
Our portfolio includes hundreds of house extensions projects across London. Here are three examples that illustrate the range of work we undertake:
Victorian Terrace, Hampstead (NW3)
A comprehensive house extensions project on a four-bedroom Victorian terrace in a conservation area. The project required careful liaison with Camden planning officers to ensure the design respected the architectural character of the street while delivering modern living standards. Completed on time and within the agreed budget, the project added approximately 20% to the property value.
Edwardian Semi, Crouch End (N8)
A family of five commissioned this house extensions project to create additional space and modernise the property while retaining its Edwardian character. Original features including cornicing, ceiling roses, and timber panelling were carefully restored, while new elements were designed in a contemporary style that complements rather than imitates the original architecture.
Period Property, Highgate (N6)
This substantial house extensions project in Highgate Village required Listed Building Consent and close collaboration with the local conservation officer. The design balanced the need for modern comfort and energy efficiency with the preservation requirements of the listed building. Specialist heritage contractors were appointed for sensitive elements including lime plastering, timber window restoration, and stone repairs.