A double-storey extension adds two floors of new space to your house in one build, typically a larger kitchen or living room downstairs with an extra bedroom, bathroom or en-suite above, sharing one set of foundations and one roof.
In Marlow, as anywhere in Buckinghamshire, the price of a double-storey extension is driven mainly by size, materials and access. Across the South East our guide sits at £67,000–£134,000 with VAT, but restricted parking, a narrow side passage or extra preparation can move the figure either way. That is why every price we give follows a visit to the house.
- Guide price
- £67,000–£134,000 inc VAT
- Time on site
- 14–22 weeks on site
- Postcodes
- SL7
- Planning rules
- England
- Local area
- Buckinghamshire
- Survey
- Free, with a written quote
Double-storey extensions prices in Marlow
Use the table to gauge the cost of a double-storey extension in Marlow before we visit. Figures include VAT and are scaled for the South East, coming out about 12% above the national guide, which gives £67,000–£134,000 overall. After a free survey, you receive a fixed written price.
| Type of job | What it usually involves | Guide in Marlow |
|---|---|---|
| Small (15–20m² total) | A compact rear or side extension adding a small room on each floor. | £45,000–£73,000 |
| Typical (30–40m² total) | A larger kitchen or living room below with a double bedroom or en-suite above. | £67,000–£134,000 |
| Large (50m² plus total) | Wide or deep extensions, often with a new bathroom, steelwork and high-spec glazing. | £112,000–£202,000 |
Your price depends mostly on:
- Combined floor area across both storeys
- How the new roof joins the existing roof
- Bathrooms, en-suites and plumbing on the upper floor
- Foundation depth, ground conditions and drains to divert
What do you get?
- Groundworks, foundations and below-ground drainage
- Insulated cavity walls with brick or render to match the house
- Roof structure tied into the existing roof, tiled to match
- Electrics, plumbing and heating by qualified trades, with certificates
- Windows, external doors, plastering and internal doors
- Building control inspections and completion certificate
Local housing and a double-storey extension
Buckinghamshire runs from Chiltern villages of brick and flint, through Metroland interwar semis around Amersham and Chesham, to Victorian terraces in High Wycombe and Aylesbury. Milton Keynes adds estates built from the 1970s onwards in brick, render and timber frame, and new housing continues to spread around its edges.
Chiltern cottages may need lime repairs and careful window choices in conservation areas. Interwar semis suit hip-to-gable lofts and rear extensions, while Milton Keynes homes often need roofline renewal, replacement doors and garage conversions.
None of this holds for every street in Marlow, which is why we look at your actual house before pricing a double-storey extension rather than assuming from its age alone.
Planning and building rules in Marlow
England, Wales, Scotland and Northern Ireland each handle planning a little differently. Marlow comes under England, covered below, and we raise any questions with the council before your quote is finalised.
In England, a two-storey rear extension can only be permitted development if it is no more than 3 m deep, at least 7 m from the rear boundary and finished to match the house. Most double-storey and all two-storey side extensions need full planning permission.
Building Regulations approval is needed, with structural calculations for beams, foundations and the roof. The Party Wall etc. Act 1996 often applies on semi-detached and terraced houses.
Rules change and every property is different, so we check the planning history and constraints for your address before we quote.
From survey to handover
We look at the job
First, someone visits your home in Marlow to see the space, take measurements and note anything that could affect the work, such as access or the condition of what is there. There is no charge for this visit.
Price and scope agreed
We put everything into a written, fixed-price quote: the specification, what's excluded, staged payments and the expected time on site. Take whatever time you need to decide; there's no pressure to commit.
Dates and permissions
Before anyone starts, we confirm the programme in writing, check whether consents are required and arrange deliveries. If a skip or scaffolding on the road needs a permit, we apply for it.
Build and hand over
During the work we protect the areas around it and tidy up at the end of each day. Once complete, we check the finish, remove our materials and hand over paperwork such as building control sign-off where it applies.
Why homeowners choose us
Questions about a double-storey extension in Marlow
Do I need planning permission for a two-storey extension?
Most two-storey extensions need planning permission, but it depends on the size, height and position, your home's location, whether it is listed or in a conservation area, and which UK nation you are in. In England, a two-storey rear extension can sometimes be built under permitted development if it is no deeper than 3m and at least 7m from the rear boundary. We check what applies before quoting.
Can the new upstairs room have its own bathroom?
A new upstairs room can usually have an en-suite, provided waste pipes can reach the soil stack or a new one can be installed. Running the waste pipe to the right falls is the main design question, and a pumped system is an option where gravity will not work. We plan bathroom positions at the design stage so pipes, ventilation and insulation are built in rather than added later.
Is a double-storey extension better value than single-storey?
A double-storey extension usually gives better value per square metre than a single-storey one, because the same foundations and roof serve twice the floor area. The overall cost is higher, and planning can be harder because of the effect on neighbours. If you might want an extra bedroom later, building both floors now avoids a second round of disruption and scaffolding.
Who do I deal with during the job?
One point of contact manages your project from survey to handover. They schedule the trades working under our contract, keep you updated on progress, and are the person to call with any question. You don't need to coordinate builders, electricians or decorators separately, because that is part of what we do.
How long will a double-storey extension take?
On a typical home, allow 14–22 weeks on site. Larger projects, older buildings and extra work found once things are opened up can extend that, while simpler jobs may be shorter. Your quote includes an estimate of time on site, and your point of contact keeps you updated on the programme as the work progresses.
Nearby towns
Whether you're in the centre of Marlow or on its outskirts, SL7 addresses are covered. We extend the same service, and the same fixed-price approach, to Maidenhead, High Wycombe and Slough.