House extensions – Double storey extension
London architect guide
Double storey extensions in London
A double storey extension can unlock extraordinary potential for your London home.
Whether you’re adding a generous kitchen-diner on the ground floor with a new bedroom and bathroom above, or extending an entire wing for family space, a two storey extension makes better use of your site while maximising value and achieving construction efficiency.
At GOAStudio architects, we’ve helped homeowners across London navigate the full process from feasibility and planning strategy to design, approvals and completion.
This guide answers the most common questions homeowners ask when considering a double storey extension project.
FOR ADDITIONAL ADVICE SEE THE FOLLOWING RESOURCES FOR YOUR HOME PROJECT
Royal Institute of British Architects (RIBA)
- Do I need an architect for my extension or home project?
- Why work with an architect?
- How do architects calculate their fees?
Planning Portal
Do you need planning permission for a loft conversion Finchley Road Camden NW6_Master bedroom double height space by GOAStudio architects
Double rear extension Tower Hamlets E1 GOAStudio architects
Top 10 Tips for Planning Your Double Storey Extension
- Start with a clear plan: what do you want to achieve — more space, more light, more value?
- Consider your whole home: how will the extension improve layout, flow, and function?
- Factor in long-term needs: growing families, working from home, or future resale?
- Understand planning context early: check if you’re in a conservation area or restricted zone.
- Engage with neighbours where possible especially if there are boundary or privacy concerns.
- Plan with your design team for how the home will function during the build i.e. temporary kitchen, access, storage.
- Don’t neglect eco-consideration about heating, ventilation, and insulation to improve comfort.
- Invest in proportion and balance: bigger isn’t always better, the space needs to work.
- Keep finishes and furnishings in mind from the start to help you visualise the space.
- Trust your architect to bring ideas and be practical with the process from start to finish.
GOAStudio London residential architecture
George Omalianakis, BA (Hons) B.Arch Pg Dip Environmental Design.
Chartered RIBA architect and ARB registered.
RIBA Conservation Registrant specialism in conservation areas, period and Listed buildings.
Residential expert contributor for magazines including Which? magazine and Ideal Home.
Last reviewed by George Omalianakis for GOAStudio: August 2026
How much does a double storey extension cost?
In London, the construction cost for a double storey extension typically ranges from £3,000 to £4,500 per square metre, depending on the scope, structural complexities, level of internal reconfiguration, and level of finishes.
While you gain more space than a single storey extension, costs don’t double for the additional space. This is because you’re already paying for groundwork, foundations, and roofing.
A typical double storey side or rear extension (20–40m² over two levels) might cost between £90,000 and £150,000+, excluding VAT, kitchen, bathrooms, and professional fees.
We help you shape your brief around realistic cost expectations and prioritise what matters most to you.
To help manage your budget, we recommend setting aside a separate allowance for kitchen and bathroom fit-outs, flooring, lighting, and decoration. These internal elements can vary widely in cost and tend to shape the final quality of the space.
We also advise including a 10–15% contingency to cover unforeseen conditions. This is particularly important in older or terraced homes where access and drainage can be more complex.
Can I build a double storey side extension?
Yes, if your home has space along the side and meets local planning criteria. Side extensions are popular in semi-detached and detached homes but may be restricted by width, boundary relationships, and daylight impact on neighbours.
Planners also assess whether the proposal maintains symmetry, streetscape rhythm, and spacing between buildings, particularly in conservation areas or on corner plots.
We’ll help you assess whether a side extension is feasible, and prepare a design that respects context while unlocking the potential of the available space in your garden.
We also advise on how to integrate the extension into your home’s existing character, whether that’s replicating original brickwork, matching rooflines, or creating a more modern contrast where this is appropriate.
A sensitive approach often increases the likelihood of planning success, especially in Conservation Areas or where Article 4 Directions apply.
Can you do a double storey extension under permitted development?
The government’s extended Permitted Development Rights allow certain types of two storey extensions to be built without a full planning application.
However the width of your garden, the distance to the boundaries, or your first floor layout might mean that a two storey extension under Permitted Development will not maximise what we could otherwise be achieved.
A poorly added extension can result into rooms with irregular proportions or with poor access to light. And we need to make sure that any extension retains the flow of the layout rather than being an awkward add-on to your home.
For this reason most double storey side or rear extensions in London will still need planning permission.
That said, we assess every site to check if PD might apply and if so, we can manage the prior approval process on your behalf.
Do I need planning permission for a double storey extension?
In almost all cases, yes especially in London. Double storey extensions raise questions of scale, massing, privacy and light that fall outside basic PD rules. Planning departments assess these projects more rigorously than single storey schemes.
We’ll review your local policies, assess what’s likely to be supported, and develop a design that works both for your goals and for approval success. We also handle all drawings, statements and application coordination as part of our planning service.
In London, borough-specific guidelines on privacy, overlooking, height and scale often determine what is considered acceptable. We prepare contextual design statements and studies where necessary to support your case. In sensitive areas, pre-application advice may be beneficial and we can manage this process to reduce uncertainty.
How long does a double storey extension take to design?
You should allow at least 6–8 months from initial design through to construction start:
– 1–2 months for measured surveys and design development
– 2 months for planning approval (plus time for revisions if needed)
– 2 months for structural design and tendering
– 2–3 months for Party Wall notices and builder availability
Larger or more complex projects may take longer especially where basement work, heritage issues or neighbour objections are involved. We’ll guide you through each stage so you know what to expect and when to make key decisions.
If your home is leasehold or within a managed development, allow additional time to obtain freeholder consent or estate approvals. Similarly, if protected trees or highways access are involved, early coordination with specialist consultants will help avoid delays. We map out all these steps clearly after our first site visit.
How long to build a double storey extension?
On-site construction typically takes 5 to 8 months depending on complexity, access, and weather conditions. Phased projects, particularly where you remain living in the house, may require more time to allow for temporary services and staged works.
We help you understand whether it’s feasible to stay in the house, and support your builder in planning a safe, efficient sequence for the build.
In terms of sequencing, builders often begin with the ground floor shell, then complete the first floor framework and roof. Internal works and fit-outs follow including bathrooms, plumbing, heating upgrades, and final decoration.
We can advise whether phasing or temporary relocation would improve delivery timescales.
How much do architects charge for a double storey extension?
Architect fees are usually higher for double storey projects due to their scale, complexity and the need for planning engagement.
Fees typically range from 9% to 12% of the construction cost, or can be agreed as fixed lump sums per stage.
At GOAStudio, we provide clear, staged proposals that include:
– Concept and design development
– Planning application and supporting documents
– Coordination with engineers and consultants
– Optional services during the build phase
You can begin with a fixed-fee consultation to explore ideas and feasibility, with no obligation to proceed further.
Our fee proposals are always tailored to your property and priorities. We can also advise on where cost-saving measures are appropriate such as simplifying roof design, reusing existing structural walls, or phasing works over two stages.
How much value does a double storey extension add?
A well-executed double storey extension often adds significant long-term value, this is especially true in London.
You’re not only increasing overall floor area, but often improving bedroom counts, bathrooms, and usability. Depending on your location this can add £100,000 to £250,000+ in resale value.
But value is also personal. If your extension creates space to grow a family, work from home, or improve day-to-day living, it pays back in more ways than one.
Adding usable square footage, especially bedrooms and bathrooms, usually places your property in a more desirable market segment.
For example, increasing from two to three bedrooms or adding a master en suite can dramatically expand your buyer pool.
We recommend that you ask local agents to validate resale value and advise on what features appeal most in your area.
Request a free Feasibility Review
If you're not quite sure what’s possible, or whether now’s the right time, we can help you make an informed start.
Send over your address, a few photos, and a short outline of what you're hoping to achieve.
We’ll take a look at your site context, planning history, and potential constraints and respond with early advice tailored to your home and circumstances.
Email us with the subject “Feasibility Review” or send us your information through our Contact us page.
Book a Home Consultation
It is the ideal first step if you are planning works to your home or if you are in the middle of purchasing a property in need of renovation.
George will visit the property to understand its character, how you use the space day to day, and offer advice on all relevant design and other matters.
You will come away with a clear sense of your next steps and greater confidence to make informed decisions. The consultation (2hrs, £250 +VAT) is a standalone service with no obligation to proceed.
Visit our Contact us page to book.
Not sure where to start with your renovation or extension?
Read this before you meet an architect, speak to a builder or spend money on drawings.
This free, architect-prepared guide walks you through the 15 essential questions every homeowner should ask before starting a project.
"Should we renovate, extend… or walk away?
15 Questions every London homeowner must ask before starting a project to include homes in Conservation Areas and Listed Buildings."
Whether you're planning a modest rear extension or major alterations to a listed home, it will help you avoid expensive mistakes and give you clarity on what’s possible for your property. Includes guidance on:
Planning approvals. Conservation areas.
Timelines and costs. Appointments you need to make.
Design and sustainability options.
When to press ahead and when to walk away
We provide architectural services for residential projects in the following London Boroughs:
Barnet, Camden, Enfield, Hackney, Hammersmith and Fulham, Haringey, Islington, Kensington and Chelsea, Lambeth, Lewisham, Newham, Tower Hamlets, Waltham Forest, Wandsworth, Westminster
We specialise in
- Greener homes
- Retrofit and energy efficiency for older and traditional buildings
- Conservation areas
- Listed Buildings
- Homeowner architectural services
- Property developments architectural services
GOAStudio London residential architecture limited.
86-90 Paul Street, EC2A 4NE, Hackney
Chestnut Avenue South, E17 9EJ, Waltham Forest
t: 0203 984 3005
