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Construction timelines for converting or extending your home

Whether your converting your loft or creating a rear extension, here are the average construction times you might be facing.

5 min read

You’ve got creative in design, been given the thumbs up in planning, found the perfect contractor and even got approval from building control. All that’s left is that all-important construction - so, how long have you got left?

Depending on the type of your project, construction can be as little as a few weeks, all the way up to a 6-month long affair. To help you get the best idea of timings, we asked one our Connect contractors - Alex from Home Republic - to give a guide to the Uk averages...

Room in loft conversion

6-10 weeks

Room in loft, also known as Velux conversions, are one of the quickest ways to add a new room to your house. Depending on the condition of your loft, they can be completed in around 2 months. This is because your contractor will need to do little to the actual structure of the space, and instead will be focused on adding new windows, sound-proofing, and insultation.

Alongside this, your team will also help fit in new floors and plaster the walls - ready for decoration. You might have a longer construction if your stairs need altering, which can be the case if they ones currently in place don’t comply with UK building regulations.

Dormer or hip to gable loft conversion

10-12 weeks

Dormer conversions are the most popular with homeowners. This is a structural extension which projects vertically from the slope of the existing roof, creating a box shape. Because this loft extension generally requires no dramatic changes and allows for the installation of conventional windows, it only requires a couple more weeks to complete with compared to a room in loft project.

If a dormer isn’t for you, you might also consider a hip to gable alternative. Ideal for end of terrace and detached homes, a hip to gable loft conversion straightens an inwardly slanted end roof to create a vertical wall. This small change can make a huge difference to the feel of the living space inside, and is becoming an increasingly popular option for homeowners. Much like the dormer, the small amount of work required to the roof keeps construction timings modest.

Mansard loft conversion

10-16 weeks

A mansard loft conversion is constructed by raising the party wall (the wall shared with your neighbours). The roof remains flat, while one outer wall slopes gently inwards. Mansards are typically found at the rear of the house, and although they are suitable for many property types, they are most popular in terraced houses.

Because this kind of loft conversion requires heavy reconstruction of the roof, along with the erection of that party wall, this is the most time-intensive. What’s more, as it requires work to the party wall, your neighbours might set restrictions on construction hours, to minimise disturbance to their household.


4-6 weeks

Much like a room in loft conversion, because your builder will be working with an existing structure, timings can be kept to a minimum. However, unlike a loft, with a garage, you’ll also have to factor in the time needed to replace your garage doors with a new wall.

Side return or rear extension

10-16 weeks

Whether you’re extending your home out towards the back of the property or utelizing some dead alleyway space to the side, you’ll be looking at similar timelines for both.

Typically, construction will be broken down into these stages:

  • Preparation
  • Foundations and groundwork
  • Ground and low-level work
  • External and internal walls
  • Building of the roof
  • Laying down the roof covering
  • Guttering, windows, doors and render
  • Knock through and plastering
  • Finishing touches, such as electrics and plumbing
  • Final tidy up

It’s worth bearing in mind, your timings will be affected by the side of your extension and complexity. For instance, if you include a floating corner or glass ceiling, these structural features will require extra time.

Two-storey extension

16-24 weeks

Two-storey extensions take a classic rear extension and use it to add another space to the upstairs of the house. Because of this extra storey, you can expect all the work involved with a rear extension, plus the work required for that second storey.

Depending on both the size and complexity of your project (see above), you could be waiting up to 6 months for construction on your home to be complete.

Converting a basement

10-14 weeks

Unlike converting a garage, a basement conversion is likely to be a big job. In 99% of cases, you’ll need to allow time for both excavations and underpinning, alongside more cosmetic elements.

What’s more, it’s likely your basement will be difficult to access, making work that little bit more tricky. While skilled contractors can navigate hard to reach basements, there will be a time delay on the work.

Retrofitting a basement

20 - 24 weeks

And lastly, should you lack a basement to convert, you’ll need to retrofit one. These are becoming more common in London, where space is at a premium. They involve excavating a new level under an existing property before a new set of rooms is added to the home.

If your home is suitable for such an extension, over the 20-24 weeks, construction will involve...

  • Preparation
  • Exposing the foundations
  • Underpinning
  • Excavation and waste disposal
  • Installing structural support
  • Reinforcement of basement floor
  • Adding damp proof membrane
  • Creating a drain waterproofing system
  • Erection of new walls
  • Adding windows and stairwell
  • Creating new suspended ceiling
  • Finishing touches, such as electrics and plumbing
  • Final tidy up

Needless to say, it’s no small project, but in areas like London, this could add insane amounts of value to your home.

Learn more about adding value.

What affects construction timings?

Of course, the timings we’ve listed here are just the UK averages. How long your own project will take will depend on a number of factors, such as…

  • Size of the project
  • Complexity
  • Location
  • Access to the construction site
  • Size of the construction team
  • Weather
  • Use of long-lead items, such as bi-fold doors and kitchens

Not to mention, whether or not your project is hit with any unforeseen delays. This could be certain materials taking too long to arrive or problems with equipment.

Want to learn more about construction? In need of some advice about a home extension? At Resi, we run a free consultation service. Simply book in a time with our team and we’ll answer all your burning project questions!

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