Timeline is the second question. Cost comes first — almost without exception — and then, once the budget conversation has settled, homeowners in Milton Keynes want to know how long they will be living on a building site.
It is a fair question and one that deserves a straight answer rather than the vague "it depends" that too many builders fall back on. It does depend — but on specific things, and those things can be explained clearly before a spade goes in the ground.
Crownhill Building Services has been delivering home extensions across Milton Keynes since 2006. Here is an honest account of how long the process actually takes, from the first conversation through to the day you get your completion certificate.
The Two Phases Most Homeowners Do Not Separate
The timeline of a home extension has two distinct parts that are easily confused. The first is everything that happens before construction starts — design, planning, approvals. The second is the build itself. Both take time and the total project duration is the sum of both, not just the weeks the team spends on site.
Understanding the difference matters because the pre-construction phase is where the biggest variation sits. A single storey rear extension that falls within Permitted Development can move from site visit to build start in four to six weeks. A double storey extension requiring a full planning application adds eight to twelve weeks before a single brick is laid. Same size of build on site. Very different total timelines.
Single Storey Extensions — Realistic Timeline
For most single storey rear and side extensions in Milton Keynes, Permitted Development applies. That removes the planning application entirely and replaces it with a lighter Prior Approval process where required — or in many cases no formal approval at all.
Pre-construction phase — four to six weeks. This covers the initial site visit, architectural drawings, structural calculations, Building Control initial notice and project scheduling. We manage all of it. You are not chasing architects or engineers separately.
Construction phase — six to ten weeks on site, depending on size and specification. A compact kitchen extension of 15 to 20 square metres sits at the tighter end of that range. A larger open-plan rear extension pushing 40 square metres sits at the wider end. Ground conditions across parts of MK — the clay-heavy subsoil common in areas like Loughton and Oldbrook — can add a few days at foundation stage, but this is factored into the programme before work begins.
If speed matters, our Spectra system reduces on-site construction to five to six weeks — without compromising on finish.
Total from first call to completion certificate: ten to sixteen weeks for most single storey projects.
Double Storey Extensions — Realistic Timeline
Double storey extensions almost always require a full planning application to Milton Keynes City Council. The statutory determination period is eight weeks from the date of validation — though in practice, straightforward applications in MK are often decided within that window.
Pre-construction phase — twelve to sixteen weeks in total. Architectural drawings and planning submission take two to four weeks to prepare properly. The council's eight-week determination period follows. Once approval is granted, structural engineering and Building Control registration add a further two to four weeks before the build can start.
Construction phase — twelve to eighteen weeks on site. Two storeys means more materials, more labour stages and more inspection points throughout the build. The foundations go in once, the scaffolding goes up once — which is why building both floors simultaneously remains better value per square metre than two separate projects phased over years. But the programme is longer and that is simply the reality of the scope.
Total from first call to completion certificate: twenty-four to thirty-four weeks for most double storey projects.
That figure occasionally surprises homeowners who were expecting something closer to a single storey timeline. The planning process is the variable that accounts for most of the difference — and it is entirely outside the builder's control once the application is submitted. What a good builder can control is the quality of the drawings submitted and the professional relationship with the local planning authority. Both reduce the risk of delays or requests for further information.
Kitchen Extensions — Realistic Timeline
A kitchen extension is in practical terms a single storey rear extension with a specific brief. The timeline follows the same pattern.
Pre-construction phase — four to six weeks. Permitted Development applies in most cases. Architectural drawings for kitchen extensions require particular care around the positioning of structural openings and the routing of services — getting this right at design stage prevents costly adjustments once work starts.
Construction phase — eight to twelve weeks. The structural element typically completes in the first half of the programme. Kitchen installation, including plumbing, gas, electrics and the fit of the units themselves, occupies the second half. CBS manages both elements under one contract. There is no handover between a building contractor and a separate kitchen fitter — which is one of the more common sources of delay and dispute in kitchen extension projects.
Total from first call to completion certificate: twelve to eighteen weeks for most kitchen extensions.
What Can Extend the Timeline
A few specific factors push projects beyond the typical ranges above, and it is worth knowing about them in advance.
Ground conditions that differ from what the initial survey indicated. Across Milton Keynes, the presence of made ground — imported fill material from the Development Corporation era of construction — occasionally shows up at foundation depth and requires a revised foundation specification. This is rare, but it does happen in parts of older MK estates.
Party wall agreements where the extension sits close to or on the boundary with a neighbouring property. The Party Wall Act requires formal notification to adjoining owners and a defined response period. We flag this at the initial site visit where it is relevant.
Building Control inspection availability. Inspections need to be scheduled at specific stages — footings, damp proof course, structural frame, roof structure, final completion. We book these in advance and build the inspection schedule into the programme.
Material lead times for bespoke elements. Bespoke bifold or sliding door systems, roof lanterns to specific dimensions or structural steel to particular specifications can carry lead times of three to six weeks. We order these at the earliest opportunity in the programme to avoid them sitting on the critical path.
Building Control and Completion
Every extension we deliver goes through Building Control from initial notice to final sign-off. The completion certificate is not a formality — it is the legal confirmation that the extension meets building regulations and the document that protects the value of the work when the property is eventually sold or remortgaged.
We manage all Building Control interactions directly. Booking inspections, responding to queries, chasing sign-offs — none of that lands on the homeowner. The completion certificate is handed over as part of the documentation pack at project close, alongside material warranties and the 12-month parts and labour guarantee that covers all our work.
How to Get an Accurate Timeline for Your Project
The figures above are realistic ranges drawn from nearly two decades of extension projects across Milton Keynes. Your specific project will sit somewhere within those ranges depending on type, size, location, ground conditions and chosen specification.
The only way to give you a programme that reflects your actual build — with dates rather than ranges — is a proper site assessment. That takes an hour, costs nothing and leaves you with a clear picture of what the project involves 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.
If cost is still the primary question, our guides on home extension cost in Milton Keynes and single storey extension pricing cover everything clearly.