Building A Granny Annexe: Everything You Need To Consider

Building a granny annexe: everything you need to consider

Granny annexes are often a practical addition to any home, providing extra living space that remains independent from your main property. In this blog, we’ve explored what an annexe is, the cost of building a granny annexe, and the most important things to consider before starting work.

What is an annexe?

In terms of your home, an annexe is a building that has an association to your main property, and shares the shares the same land. It may be interconnecting, with access to the main property through internal doorways and corridors, or detached with access via an external entrance.

Why build a granny annexe?

Self-contained annexes are referred to as granny annexes/granny flats. They earned their name because they offer a convenient way for families to accommodate elderly relatives while maintaining a level of independence.

However, of course, they aren’t reserved only for grandparents, and can also be used as a guest house or provide independence for grown-up children who are saving up for a home of their own. Not to be confused, a garden annexe is often dependent on the main property, and, rather than housing aging parents, usually serves simply as extra living space.

What is the cost of building a granny annexe?

Because a granny annexe is a self-contained building, with its own bathroom, kitchen, and bedroom, it’s important to include the costs of plumbing, electricity, and furnishings in your budget.

Of course, costs vary, depending on the materials you use, the extravagance of your furnishings, and how complex it is to install any utilities, as well as how much space you have and whether you’re planning on converting existing space or building a granny flat from scratch. With no granny annexe grants available, you’ll have to fund the construction yourself, so we’ve outlined the basics to give you an idea of your potential outlay:

  • Building a granny annexe ~ £80,000

Building an annexe for an elderly relative from scratch gives you the scope to perfectly complement the layout of your current property. However, invariably, this means they generally cost the most money of any type of annexe building.

  • Installing a prefabricated annexe ~ £30,000

Prefabricated annexes are pre-assembled and arrive ready to construct. This makes them quicker to put together, but can be limiting in terms of granny annexe designs. For instance, they are often scaled according to set measurements, rather than bespoke instructions. As such, they are generally a cheaper option than extending.

  • A garage-annexe conversion ~ £8,000

The cheapest way to create a granny annexe is to use the space already on offer within your property – for instance, your garage. This will likely already be connected to your property, allowing easy and direct access to the main home, and you can create a self-contained studio layout to ensure complete privacy and independence.

How much value will an annexe building add to my home?

An annexe building provides additional practicality to your property, offering extra living space for friends and family. You can even rent it out to a lodger, to earn a little extra income. With this in mind, of course granny annexe designs can greatly improve the value of your home. In fact, depending on the square footage, you can even fetch a further 20-30%.

Things to know when building an annexe for an elderly relative

Council tax on granny annexe

Usually, extensions to your property don’t impact how much council tax you’re required to pay. But granny flats and annexes are different – they’re built as permanent accommodation, separate to your main property. Therefore, you might be liable for increased or additional council tax payments. However, discounts (usually 50%) are often issued when all of the following apply:

  1. The granny annexe building is occupied by a relative of the homeowner
  2. The annexe building forms part of the main property
  3. The annexe is the permanent home of the person who lives there

Additionally, there are situations where you can apply to be exempt from making council tax payments on your annexe building. These include:

  • If the person living there is considered a dependent (over the age of 65/has a physical or mental disability)
  • If the person living there is under the age of 18
  • If the annexe building is unoccupied

Are granny flats worth it?

It’s common for elderly relatives to move closer to their families as they get older and more reliant on extra care – especially after the death of a spouse. At this point, you have three main choices: pay for a carer, extend, or move to a bigger house.

If you’d rather keep your aging parent or grandparent close by, and don’t want to upheave your own life, building a granny annexe can be financially beneficial over the long term and well-worth the initial expense; as highlighted previously, adding extra living space to your property can even improve its value by up to 30%.

And if your retired relative is selling their home to move closer to you, they may even be able to contribute to the cost through their house sale. if you’re relying on the funds from their property to get started, though, consider exploring quick house sale solutions that puts money in your pocket much quicker than if you sold through an estate agent.

Granny flat planning permission

Generally speaking, any structure intended to be lived in (including a granny annexe) requires planning permission, which is granted by the local council. This is to certificate the safety and lawfulness of the accommodation.

As far as building granny annexe regulations go, if your land covers up to 900m2, your structure must be built more than 3m from the back of your property and at least 0.9m from your side boundaries. For land covering more than 900m2, the distance between building work and your property boundary increases.


Self-contained granny flats offer a practical solution to elderly relatives who begin to require more regular care, so hopefully we’ve offered helpful guidance around building or adding one to your home. For even more property advice, check out the latest over on our blog.

We are proud members of...

  • NAPB
  • RICS
  • The Property Ombudsman
  • Trading Standards

We are proud to be the most regulated property buyer operating in the ‘Quick House Sale’ industry. We are an active member of the NAPB (National Association Of Property Buyers) and are RICS regulated, which means you can have every confidence of selling your home with us quickly & easily.