A double storey extension is a different kind of decision to a single storey rear extension. The scale is larger the planning picture is more involved and the return — in terms of added space and added value — is considerably greater. It is also the project that homeowners in Milton Keynes tend to think about for longer before acting on. This guide is intended to make that thinking more productive.
Crownhill Building Services has been delivering double storey extensions across MK since 2006 — in Newport Pagnell Wolverton Loughton Westcroft and the wider Milton Keynes area. Our most recently completed project was a double storey front extension in Newport Pagnell — a project that added ground floor living space and a full upper floor bedroom while matching the original brickwork of the property exactly.
What a Double Storey Extension Actually Involves
The defining feature of a double storey extension is that it adds floor area on two levels in a single build programme. Ground floor gains — an extended kitchen living room or dining space — sit beneath first floor additions that typically provide a new bedroom en-suite or bathroom. Both floors go up at the same time which makes the project more efficient than two separate single storey builds phased over years.
The structural demands are greater than a single storey project. The foundation specification carries more load. The structural frame needs to be engineered accordingly. Roof design at the junction between the new and existing structure requires careful detailing. None of this is unusual for an experienced team — but it is the reason that choosing the right builder matters more on a double storey than on a smaller project.
Does a Double Storey Extension Need Planning Permission in MK?
In most cases yes. Double storey rear extensions that project more than 3 metres beyond the rear wall of a semi-detached or terraced property — or more than 4 metres on a detached — fall outside Permitted Development and require a full planning application to Milton Keynes City Council.
The statutory determination period is eight weeks from the date the application is validated. Straightforward applications on standard residential plots in MK are typically decided within that window. We manage the full planning process — architectural drawings submission and liaison with the council — as part of the project. You are not chasing a separate architect or dealing with the planning department directly.
Front extensions — like our recently completed Newport Pagnell project — almost always require planning permission regardless of scale. The planning policies that govern front extensions are more restrictive than those covering rear additions because of the impact on the street scene. Read our full guide on planning permission for home extensions in Milton Keynes for more detail.
How Much Does a Double Storey Extension Cost in Milton Keynes?
Double storey extensions are priced on the same per square metre basis as single storey projects — but the total footprint is calculated across both floors. A double storey rear extension measuring 5 metres wide by 4 metres deep adds 20 square metres on the ground floor and 20 square metres on the first floor — 40 square metres of new floor area in total.
At our current rate that puts the starting cost for a project of that scale at £100,000 including VAT. Specification choices — roof type door and window specification internal finishes — move the figure upward from there. A full bathroom or en-suite on the upper level adds to the cost. Structural steel requirements determined by the span of the new openings are priced accurately once we have assessed the site.
Every project is quoted at a fixed price agreed in writing before work starts. No additions through the build. No surprises on the final invoice. See our full home extension cost guide for Milton Keynes for a broader picture of how extension costs break down across different project types.
How Long Does a Double Storey Extension Take?
The planning process adds eight to twelve weeks before construction can begin. Pre-construction work — detailed drawings structural engineering Building Control registration — adds a further two to four weeks on top of that. Construction itself runs to twelve to eighteen weeks on site depending on the scale and specification of the project.
Total from first call to completion certificate: most double storey extensions in MK complete in twenty-four to thirty-two weeks. That is longer than a single storey project — but the planning process accounts for most of the difference rather than the build itself.
Is a Double Storey Extension Worth It in Milton Keynes?
The value case for a double storey extension in MK is straightforward. Adding a bedroom and bathroom at first floor level alongside extended ground floor living space increases the property's market value and its appeal to buyers. For families who need more space but do not want to move — with all the costs that entails — it is consistently the most cost-effective route to a larger home.
The Newport Pagnell project referenced above is a good example of what the finished result looks like when the design is handled carefully. The brickwork matches. The proportions work. The street presence of the property is improved rather than compromised. That outcome is not accidental — it comes from the same team doing this work since 2006.
Next Step
A free site visit is the starting point. We look at the property assess the ground check your planning position and give you an honest picture of what is possible and what it will cost — before you commit to anything.
Get in touch today to arrange your free no-obligation consultation. Take a look at how we run our projects or browse completed extensions across Milton Keynes to see the standard of finish we deliver.