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Garden room extension cost: breakdown and ideas to keep it under control

Let's breakdown the cost of a garden room extension with this guide covering materials, contractors, planning and more.

5 min read

A garden room is more than an extension of your home, the multitude of design options and features available can make it an extension of you and your family. Usually a more affordable option, a garden room is a cost-effective alternative to traditional house extensions.

Garden room extensions can add value, increase living space and breathe new life in a property by installing a unique place that’s separated from the rest of the house. But while adding a garden room can add value to your home and your overall lifestyle, creating this space requires an adequate budget.

When making the decision to build a garden room, it’s important to understand the costs upfront.

How to calculate the cost of a garden room extension

Let’s start by examining the factors that affect the cost of a garden room extension.

  • Complexity
  • Type of contractor used
  • Size
  • Type and quality of materials used
  • Prep work
  • Location of property


Because a garden room can be anything from a simple space to relax to an open plan, annexe, complete with bifold doors and a living roof on top, the prices can vary massively - especially when you’re working with complicated structures or unique features.

Complicated structural elements and design elements require skilled professionals and additional time. The skill level and experience of the contractor will affect costs, but the outstanding results are priceless.

Contractor type

You’ll find that builders/contractors tend to come in three varieties…

  • Small, one-man bands
  • Medium, locally established teams
  • Large, high-quality companies

The bigger the contractor company and the better the quality, the more you’ll have to pay during construction. While it can be tempting to cut corners and opt for a one-man band, you’ll find these smaller teams tend to be prone to delays, bad paperwork, and ultimately can put your project at risk. For the best results and return on your investment, it’s worth splashing out on a reliable, experienced team of builders.

Estimated construction costs

Basic build: using a one-man contractor: £1500/m2

Standard, high-quality build: medium-scale contractor: £1650/m2

Premium quality build: large scale building contractor: £1800/m2

*Keep in mind that these estimates do not include VAT.

The more skilled and experienced the contractor, the greater the cost, but this is certainly not a place to cut corners. Quality contractors can make the difference between a good outcome and an excellent one.

We understand that finding the right contractor can be a daunting experience, which is why we have created an essential tool to help you navigate through the selection process.


The largest factor involved in building a garden room extension is the cost of materials and labour. While there are several factors that go into the final cost of a project, smaller, less complex builds will inevitably cost less than those with larger square metres involved.

A garden room in London with a living wall

Type of materials used

Depending on building material costs and the price of installation, costs can vary considerably. Higher-end materials - like granite and stone countertops, bespoke flooring and fixtures, and luxury lighting - can increase the overall cost substantially. Window and door materials can also affect the cost, such as double glazing, bay windows or window seats.

Remember: the best investment is always going to be your build, rather than your fittings. A glass ceiling or beautiful exposed brick wall will bring more value to your property than some bespoke kitchen cabinents.

Prep work

Another factor to consider is the cost of prep work involved with some installations. Custom fixtures and artisan features that take substantial prep time can add to the cost estimate. Hiring an architect, while adding to the cost, helps streamline the project. Architects can design a space that most closely suits your requirements at the outset, saving costs to make changes later. They can also secure optimal quotes from contractors, source the most cost-efficient materials and aid with the entire prep process.

Other prep work could also affect cost. Removing trees and shrubbery; removing contaminated materials, like asbestos; dismantling awnings and removing existing structures, like decks and patios, might need to be done before any work can begin. Depending on the extent of prep work, costs can increase.

Location of property

Depending on where your home is located, the cost may be affected. Builders in London and the South East of the UK typically charge more than those in other locations. While sourcing locally is the best option, transportation and shipping factors due to a property’s location will also affect the cost of the project.

Other costs to consider

While your initial estimate will include the factors mentioned above, there could be other budget factors that affect the final cost of the project.

  • Planning cost
  • Delivery cost
  • Glazing costs
  • Electrical connection
  • Insurance

Planning costs

During planning, we’ll help you apply for one of two things: planning permission or a lawful development certificate, and depending on which scenario, planning costs may differ.

Delivery costs

The cost for delivering supplies to the construction site will vary depending on the location of the property and whether or not materials will need to be shipped, and from how far.

Electrical connection

Electrical connection should always be performed by a skilled professional. Electrical contractors charge according to the complexity, size and location of the project. As in choosing a general contractor, skill and experience affect the cost of the electrician.

Glazing costs

Glazing refers to the glass used, and the more glass you use, the higher the costs of materials and installation. You’ll also see prices increase depending on the type of glass required or desired. Structural glass will have the biggest price tag, while you may also consider investing in triple-glazing too or solar reflective glass treatments.

Insurance costs

It is important to update your property insurance policy prior to starting the build, as your current policy only covers your home as it stands right now. Our team will scout out the best insurance options so your home is protected now and in the future.

A garden room in London, completed in 2015 with bifold doors

Budgetting with Resi

Though it is probably the most daunting step in the overall decision-making process, breaking down the budgetary factors is critical. The good news is, when you work with Resi, navigating the financial waters is one less thing to worry about. Because we have our own team of financial experts, our team offers a unique combination of housing know-how and practical funding expertise.

Our team will work closely with you to define what budget you’ll require, and assist you with financing. Resi Finance is our in-house finance team, with access to over 90 lenders to ensure you get the right advice. To chat with one of our financial experts about a cost estimate or other budgetary concerns, contact us today to get started.

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