Granny Annexe vs Home Extension: Costs and Benefits Compared

If you need extra living space for a family member, there are really two main routes: build a granny annexe or garden annexe in the garden, or extend the main house. Both can work — but they solve the problem in quite different ways, with different costs, timelines and long-term implications. Here's how they compare.

We build granny annexes and garden annexes for families across Essex and Hertfordshire, as well as the rest of the UK, and this is a comparison we talk through with clients regularly — because the right answer genuinely depends on your circumstances, not just your budget.

The core difference

A home extension adds space by building onto your existing house — typically a new room, or an extra storey, connected directly to the main living areas. A granny annexe or garden annexe is a separate, self-contained building in your garden, with its own entrance, kitchen, bathroom and living space.

The extension keeps everyone under one roof, sharing the same front door. The annexe gives genuine independence — separate space, separate entrance, and often, separate legal treatment.

Cost comparison

Costs vary hugely depending on size, specification and location, so treat any figures as a general guide rather than a quote — but broadly:

• Extensions are often priced per square metre similarly to an annexe, but you also need to factor in disruption to your existing home, potential need for temporary alternative accommodation during the build, and the complexity of tying new structural work into an existing building.

• Granny annexes and garden annexes are typically built as a self-contained, one-off structure — meaning the build often has less disruption to daily life in the main house, since work is largely contained to a separate part of the garden.

Extensions can sometimes work out cheaper per square metre for straightforward single-storey additions, but the "hidden" costs of extended disruption, temporary living arrangements, and structural complexity often narrow that gap. A proper quote and site survey for both options is the only way to get an accurate comparison for your specific property.

Planning permission

This is often the deciding factor for many families.

• Extensions frequently require full planning permission, particularly for anything beyond a modest single-storey addition, or if your property has specific restrictions attached. Even where permitted development rights apply, there are still size and design limits to work within.

• Granny annexes and garden annexes, as we've covered in previous posts, are usually built under the Caravan Act, requiring only a Certificate of Lawful Development — a simpler, generally faster process than full planning permission.

If speed and planning certainty matter to you, an annexe often has the edge.

Independence and privacy

This is where the two options diverge most in terms of lived experience, rather than just cost.

An extension keeps family close, physically connected — which some families want, particularly if a parent needs regular support or supervision. But it also means less privacy and independence for everyone involved; noise, visitors, and daily routines are all shared more directly.

Granny Annexe built in Maldon, Essex

A granny annexe or garden annexe offers a genuinely separate home — its own front door, its own space, its own routine — while still being close enough for family to be there in minutes, not hours. For many elderly parents in particular, this balance of independence and proximity is exactly what makes an annexe more appealing than moving in under one roof.

Property value

Both options can add value to your property, but they do so differently.

• A well-executed extension increases the size and specification of your main house, which can boost its overall value and appeal to future buyers looking for a bigger family home.

• A granny annexe or garden annexe adds a self-contained secondary living space — which some buyers see as a major asset (multi-generational living, home office flexibility), while others may be less interested if they don't have a use for it. It's worth discussing with a local estate agent how annexes are viewed in your specific area before assuming either option is automatically the better investment.

  
  

Build time

Extensions can take many months, particularly if structural work, planning permission, and building regulations approval are all involved — and there's often an element of living alongside an active building site.

Granny annexes and garden annexes are generally quicker to complete, since they're a self-contained build separate from your existing home, without the complexity of tying into existing structures and services. Timelines still depend on the scale and specification of your project, so it's worth discussing realistic timescales for your particular build during your consultation.

Which is right for you?

Consider an extension if:

• You want everyone under one roof

• Your family member needs frequent, close supervision or support

• Your garden space is limited but your house has room to extend

Consider a granny annexe or garden annexe if:

• Independence and privacy matter — for you and for the family member moving in

• You want a faster, generally simpler planning process

• You have garden space to work with

• You're thinking about flexible future use (adult children, home office)

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The bottom line

There's no universally "better" option — it depends on your family's needs, your garden and house layout, and what you're trying to achieve. Many families are surprised to find that a granny annexe works out more practical, and often more cost-effective once disruption and planning timescales are factored in, than they initially assumed.

Weighing up a granny annexe against extending your home? Get in touch with Mobile Annexe Ltd for a free consultation. We build granny annexes and garden annexes across Essex, Hertfordshire and the rest of the UK, and we're happy to talk honestly through whether an annexe is the right fit for your family — not just sell you on the idea.

📞 07770 365734 ✉️ enquiries@mobileannexe.com