When you need more space, and specifically, independent space, for a parent, an adult child, or paying guests, a granny annexe is usually one of the first things that comes up. What doesn’t get talked about enough is that “granny annexe” actually covers two very different projects: buying a bespoke, fully finished unit from a specialist, or building one yourself. I’ve been working through exactly this decision for my own family, and the gap between those two routes is bigger than most cost guides let on.
Why I Started Looking at this
The trigger for me was wanting to keep my dad close by and properly supported as he’s got older. A retirement home nearby was on the table for a while, but it never felt right, and once I started running the numbers on ongoing care costs versus a one-off build, a self-contained annexe in the garden made a lot more sense. That’s the lens I’m looking at this whole decision through, not as an abstract investment question, but as something I’m actually in the middle of building.
Option One: Buy it Bespoke
If you don’t have the time, tools or inclination to build it yourself, going with a specialist manufacturer is the straightforward route. You get a fixed price, a fixed timeline, and someone else managing building control and installation. Timberspecs’ bespoke granny annexes are a good example of what this looks like in practice, fully insulated, designed for permanent living, with prices starting from around £99,845 for a bespoke build. That price reflects everything being handled for you, design, manufacture, delivery and fit-out, which is worth a lot if your time is limited or the project needs to move fast.
Option Two: Build it Yourself
The other route, the one I’ve gone down, is self-building. It’s slower and takes more of your own labour and time, but it gives you far more control over spec, layout and where the money actually goes. I’m building mine from a timber frame, and doing the labour myself is the single biggest reason my costs are coming in well under what a bespoke, fully-fitted equivalent would cost.
It’s not the right call for everyone. Self-building means managing your own planning application, building control sign-off, and every trade involved, electrics, plumbing, drainage, insulation, on top of the actual construction. If you’re seriously considering this route, I’ve written up everything I learned researching my own annexe build, planning permission, Building Regulations, insulation for year-round use, and how to get water, electrics and drainage into the garden, in my full garden room annexe guide. It’s worth reading before you commit to either route, since it’ll give you a realistic sense of what’s actually involved once you get past the initial idea.
Why the Annexe Route Made More Sense Than Extending, for me
I did also look at extending the house rather than building in the garden. Extensions add solid, well-evidenced value. Nationwide’s House Price Index special report from October 2025 found that adding a large double bedroom and bathroom can increase a three-bedroom house’s value by as much as 24%. But an extension is fused to the house permanently. It adds space, but not flexibility.
An annexe stays useful as circumstances change in a way an extension can’t. Right now, mine is being built with my dad in mind, but the same space could just as easily become a home office, guest accommodation, or something my kids use one day. That flexibility mattered more to me than the raw square footage.
The Care Cost Angle
This is the calculation that actually tipped it for me. UK residential care fees run well into five figures a year for a lot of families, and once you look at that as an ongoing cost rather than a one-off, a self-built annexe starts to look less like an expense and more like a long-term saving. It’s not a decision everyone needs to make on financial grounds, but it was a big part of mine.
A Few Practical Things Worth Knowing Before You Commit
- Planning permission applies differently depending on the annexe’s size, position and intended use, always check with your local authority before assuming either route is simpler.
- Disruption is much lower with an annexe build than an extension, since it largely happens away from the main house.
- Resale appeal is narrower for an annexe than an extension, it suits multigenerational families, rental income seekers, or people planning ahead for care, rather than every buyer.
Final Thoughts
There’s no single right answer between buying bespoke and building it yourself, it comes down to your budget, your timeline, and how much of the process you want to take on personally. If you’d rather have it handled for you, a specialist route like Timberspecs makes sense. If you’re willing to put the time in yourself, it can bring the cost down significantly, and if that’s the direction you’re leaning, my full annexe build guide is a good next stop before you start.
