How Much Does a Granny Annexe Cost? A Pricing Guide
"How much does a granny annexe cost?" is usually the very first question we get asked — understandably, since it's often the factor that determines whether the project goes ahead at all. The honest answer is: it depends on quite a few variables, but there are clear cost factors that let you build a realistic budget before you start.
We build granny annexes and garden annexes for families across Essex and Hertfordshire, as well as the rest of the UK, and this guide walks through what actually drives the price up or down, so you can plan with confidence.
What determines the cost of a granny annexe?
Size
This is the single biggest factor. A compact studio annexe will cost significantly less than a larger one-to-five bedroom build. Since granny annexes are generally built under the Caravan Act, size is capped at up to 136m² — but within that range, cost scales fairly directly with square metreage.
Specification and finish
Kitchen and bathroom specification, flooring, cladding choice, heating systems, and interior finish all affect the final price considerably. A basic, practical specification will cost less than a high-end finish with premium fittings throughout — and it's worth deciding early on where you want to prioritise spend, since this is usually where budgets move the most.
Groundworks and site conditions
Every plot is different. Site preparation, foundations, and the complexity of getting power, water and sewage connected all vary depending on your garden's layout, access, and existing infrastructure. A straightforward, accessible plot with services nearby will generally cost less to prepare than a more challenging site.
Access
If materials and equipment can be easily brought onto site, the build tends to be more cost-effective. Restricted access — narrow side passages, limited vehicle access, or having to work around existing structures — can add to both cost and build time.
Planning route
Most granny annexes fall under the Caravan Act, needing only a Certificate of Lawful Development — a simpler and generally less costly process than full planning permission. If your project instead needs full planning permission (for example, due to size or specific site restrictions), that adds both cost and time to the overall project.
Design complexity
A straightforward, standard layout is more cost-effective than a highly bespoke design with unusual room configurations, or non-standard architectural features.
What's typically included in a granny annexe build?
When budgeting, it helps to think about the full scope of what a proper turn-key annexe build should include:
Site survey and groundworks
Foundations
Structural build
Doors, windows and glazing
Kitchen and bathroom fittings
Electrics, heating and lighting
Flooring and interior decoration
Connection of power, water and waste services
Planning application (Certificate of Lawful Development)
Getting a clear, itemised quote that spells out exactly what's included — and what isn't — is one of the most important steps in comparing options, since headline prices can look very different depending on what's bundled in versus quoted as an extra.
Ongoing costs to budget for
Beyond the build itself, it's worth factoring in ongoing costs once your annexe is complete and occupied:
Council tax. As covered in our previous post, most annexes need their own council tax band once occupied, though many qualify for a 50% discount.
Utilities. Depending on how services are set up, you may need separate or shared utility arrangements with the main house.
Insurance. A self-contained annexe typically needs its own appropriate cover, distinct from the main house policy.
Maintenance. Like any building, ongoing upkeep should be factored into your long-term budgeting.
How does cost compare to a home extension?
As we covered in a previous post comparing the two options, extensions and annexes can end up in a broadly similar cost bracket per square metre, but annexes often avoid some of the "hidden" costs associated with extensions — disruption to daily life, potential need for temporary accommodation, and the complexity of tying new structural work into an existing building.
Getting an accurate quote for your project
Because so much depends on your specific plot, size requirements and specification choices, general figures can only take you so far. The most reliable way to understand what your granny annexe or garden annexe will actually cost is a proper site survey and consultation — which lets us assess your garden, talk through your requirements, and put together an accurate, itemised quote rather than a generic estimate.
Financing your annexe
Many families fund a granny annexe through savings, remortgaging, specific home improvement finance, or a combination of these. It's worth having an early conversation with your mortgage provider or financial adviser about what's realistic for your circumstances before finalising your design and specification.
The bottom line
Granny annexe costs vary considerably depending on size, specification, site conditions and planning route — which is exactly why a proper site survey and detailed, itemised quote matters far more than any generic price you'll find online. Understanding the full picture, including ongoing costs, will help you budget realistically from day one.
Typically, one of our custom built granny annexes start from £80,000, and will rise to over £200,000 for the largest 20m x 6.8m garden annexe.
Want an accurate cost for your granny annexe or garden annexe project? Get in touch with Mobile Annexe Ltd for a free consultation and site survey. We build across Essex, Hertfordshire and the rest of the UK, and we'll put together a clear, itemised quote based on your actual garden and requirements.
📞 07770 365734 ✉️ enquiries@mobileannexe.com
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