How Long Does a House Extension Take to Build? | Basildon Builder’s Guide
If you’re thinking about extending your Basildon home, one of the first things you’ll want to know is how long the whole process takes. It’s a reasonable question with a frustrating answer — it depends. The type of extension, whether you need planning permission, your property’s ground conditions, and how well the project is managed all play a role. But while every project is different, there are typical timescales for each stage that give you a reliable framework to plan around.
This guide walks through the full timeline of a house extension, from the initial design conversations through to handing over the keys to your new space. The timescales are based on the kind of projects we build across Basildon, covering everything from modest single storey rears to larger double storey and wrap-around extensions.
The Pre-Build Phase: Design and Approvals
Before any building work starts, there’s a period of design, approvals, and preparation that sets the foundation for everything that follows. Rushing this stage is one of the most common reasons extensions run into problems later, so it’s worth investing the time to get it right.
The design phase typically takes two to four weeks. This involves an architect or designer visiting your property, taking measurements, discussing what you want to achieve, and producing drawings. For a straightforward rear extension on a semi in Laindon or Vange, the design might come together quickly. For a more complex wrap-around on a detached property in Langdon Hills or Billericay borders, allow the full four weeks to explore options and finalise the layout.
Once the design is agreed, the approvals process begins. If your extension falls within permitted development rights, you can skip the full planning application. Under current rules, you can typically extend three metres from the original rear wall for an attached house or four metres for a detached property without planning permission. Larger extensions up to six or eight metres may be possible through the prior approval process, which involves notifying the council and your neighbours and takes 42 days.
If you do need full planning permission — which is common for double storey extensions, side extensions in certain locations, or properties in conservation areas — Basildon Borough Council has eight weeks to determine a standard householder application. In practice, allow ten to twelve weeks from submission to decision to account for validation time and any requests for additional information or minor amendments.
Building Regulations approval runs alongside or follows the planning process. A full plans application typically takes four to six weeks for approval. Alternatively, your builder can work under a building notice, which allows construction to start sooner with inspections happening as the work progresses.
Altogether, the pre-build phase takes anywhere from six weeks for a straightforward permitted development project to four months or more where full planning permission is required. It feels like a long time when you’re eager to get started, but every week spent on proper preparation saves you time and money during the build itself.
Single Storey Extensions: Eight to Twelve Weeks
A single storey rear extension is the most common type we build across Basildon, and the most predictable in terms of build time. For a typical three to four metre rear extension on a semi-detached or terraced house, expect eight to twelve weeks from breaking ground to completion.
The first two weeks cover groundwork. This means excavating the foundation trenches, pouring concrete, and building up the substructure to damp proof course level. Groundwork is the stage most affected by weather and ground conditions. Basildon sits largely on London clay, which can present challenges during wet periods — trenches fill with water, clay swells, and in some cases deeper foundations are needed to account for soil movement. A spell of heavy rain during this stage can add a week to the programme.
Weeks three and four see the brickwork rising to wall plate height. Steel beams go in where internal walls are being knocked through, and the structural opening between your existing house and the new extension takes shape. By weeks five and six, the roof structure is on and the extension is made watertight. This is a key milestone because from this point the internal trades can work regardless of what the weather is doing outside.
The remaining weeks cover first fix electrics and plumbing, insulation, plastering, and then the second fix phase — fitting the kitchen if it’s a kitchen extension, laying flooring, tiling, hanging doors, decorating, and all the finishing touches that turn a building site into a living space. This phase involves the most trades working in sequence, and a builder who manages this programme well keeps everything moving without gaps between one trade finishing and the next starting.
Larger single storey extensions — six to eight metres deep, or wrap-around designs that extend to the side as well as the rear — add two to four weeks to these timescales simply because there’s more of everything to build and finish.
Double Storey Extensions: Twelve to Sixteen Weeks
A double storey extension provides significantly more space but doesn’t take twice as long as a single storey because the foundations, walls, and much of the structural work serve both floors simultaneously. For a typical double storey rear extension on a three or four bedroom property, expect twelve to sixteen weeks from foundations to completion.
Groundwork takes slightly longer because the foundations need to be deeper and wider to carry the additional load of two storeys. Brickwork takes longer to reach full height, and scaffolding is needed from an earlier stage. The roof is more complex, often involving tying the new ridge line into the existing roof and ensuring the junction is properly weathertight. First floor joists, any staircase modifications, and additional structural steels for the upper level all add time that a single storey doesn’t require.
The internal finishing phase is longer too, simply because there are more rooms to plaster, wire, plumb, and decorate. If the first floor includes a new bathroom or ensuite, the plumbing, waterproofing, and tiling for that adds at least a week compared to bedrooms alone.
Properties across Basildon vary in age and construction, and both affect the build timeline. The post-war housing in areas like Fryerns and Barstable was built quickly and can sometimes throw up surprises when you open up walls or dig foundations next to existing footings. The newer estates around Nether Mayne and North Benfleet tend to be more straightforward structurally but may have different ground conditions that affect foundation design.
What Causes Delays?
Even well-managed projects encounter setbacks, and understanding the common causes helps you plan realistically. Weather is the biggest factor during groundwork and brickwork. Extended wet periods in autumn and winter can add one to three weeks to these stages. Once the roof is on and the extension is watertight, weather has little impact on progress.
Ground conditions are unpredictable. If the foundation trenches reveal filled ground, tree roots, old drains, or ground that’s softer than expected, additional work is needed before building can continue. Building control may require deeper foundations or engineered solutions that weren’t anticipated, which adds both time and cost.
Material lead times catch some projects out. Certain bricks — particularly if you’re matching the existing house — can have lead times of four to eight weeks. Roof tiles, structural steels, and kitchen units also have variable availability. A builder who plans ahead orders these items well before they’re needed on site, but delays in the supply chain can still cause gaps in the programme.
Specification changes during the build are another common cause of delays. Deciding mid-project that you want a different kitchen layout, additional windows, upgraded flooring, or different bathroom fittings means pausing work while new materials are sourced and potentially revisiting work that’s already been done. The more decisions you finalise before construction starts, the smoother and faster the build runs.
How to Keep Your Extension on Track
There are practical steps you can take to help your project stay on schedule. Finalise all design decisions before the build starts. That means kitchen design and supplier, bathroom specifications, socket and lighting positions, flooring choices, tile selections, and paint colours. Your builder needs this information to programme trades in the right sequence and order materials with enough lead time.
Respond to questions quickly during the build. Decisions and approvals come up regularly, and a delay of a few days on your side can push a trade back by a week or more if they move on to another job while waiting.
Choose a builder who actively manages the programme. The most common reason extensions overrun isn’t the work itself — it’s poor coordination between trades. A builder who schedules every stage in advance, orders materials ahead of time, and communicates clearly with you about what’s happening each week will deliver your extension faster and with far less stress than one who reacts day by day.
Be realistic about living through the build. Most Basildon families stay in the house during an extension. The groundwork and brickwork stages are mostly external and manageable. The disruptive period comes when internal walls are removed and rooms like the kitchen are temporarily out of action. Your builder should tell you exactly when this will happen and for how long, so you can prepare accordingly.
Total Timescales at a Glance
As a rough guide, from breaking ground to completion you’re looking at eight to twelve weeks for a standard single storey extension, twelve to sixteen weeks for a double storey, and up to twenty weeks for larger or more complex wrap-around projects. Add the design and approvals phase on top — six weeks at best, up to four months if full planning is needed — and the total timeline from first conversation to moving furniture into your new space is typically five to nine months.
If you’re considering extending your Basildon home, the best starting point is a conversation with a builder who can visit, understand what you want, and give you an honest timescale alongside a detailed quote. We’re happy to do exactly that, free of charge and with no obligation. Get in touch and let’s get your project moving.