Tel 0800-619-1855Tel 0800-619-1855Modular garden rooms and granny annexes are largely manufactured away from the property and delivered in complete or near-complete sections for assembly on site. They can offer a fast route to self-contained garden accommodation, though standardised dimensions, delivery access and design flexibility vary considerably between suppliers.
BlueBee Annexes doesn't manufacture fixed modular units. We design bespoke SIP garden annexes, built from precision-engineered panels and assembled on your property to your own specification. Below, we cover what a modular garden room or granny annexe actually involves, then look at where a bespoke SIP annexe can offer a better fit.
A modular garden room or granny annexe is built off-site in a factory, in one or more complete sections (modules), then transported to your garden and joined together or positioned as a finished unit. Most of the construction, including internal fittings, is completed before the annexe ever arrives - on-site work is largely limited to groundworks, positioning and connecting services.
This is different from a fully bespoke annexe, where the structure is designed and built specifically around your garden, access and requirements rather than assembled from a standard module range.
"Modular" and "prefabricated" (or "prefab") are often used interchangeably to describe factory-built garden rooms and annexes delivered as complete sections. SIP (Structural Insulated Panel) construction is a different method again: panels are precision-manufactured off-site, then assembled into a bespoke structure on your property, rather than delivered as fixed pre-built modules. If you're specifically researching prefab annexes, our prefabricated granny annexe guide covers that in more detail.
Because modular units arrive largely complete, they typically need lorry access to the site and, in many gardens, a crane to lift the module (or modules) into final position. Where a building is delivered in two sections to comply with the Caravan Act, these are bolted together on site and the joints sealed once positioned.
This makes garden access one of the first things to check before choosing a modular option - a narrow side entrance, overhead cables, or a garden with no direct road access can rule out some modular units entirely, or add significant cost for special access equipment.
Bespoke SIP annexes are built from individual panels rather than one large pre-assembled unit, which can make them a practical option where full module access isn't available, since components can potentially be brought through more restricted spaces and assembled on site.
It depends on the specifics of your project, and you should always confirm your position with your local planning authority before proceeding. Broadly, an annexe built as living accommodation for a family member may qualify for permitted development rights in some circumstances, or be built under the Caravan Act (formally the Caravan Sites Act 1968) without full planning permission - provided it meets size limits (no longer than 20m, no wider than 6.8m, no taller than 3.05m), is ancillary to the main house, and is occupied by a family member rather than let out or used for business.
These rules have genuine exceptions and local variation, so treat this as a starting point rather than a final answer. Our granny annexe planning permission guide and Caravan Act guide go into this in more depth.
| Consideration | Modular annexe | Bespoke SIP annexe |
|---|---|---|
| Manufacture | Complete units built off-site | Precision panels manufactured off-site, assembled on your property |
| Garden access needed | Usually requires lorry and/or crane access for the full module | Components can often be brought through more restricted access |
| Design flexibility | Limited to the supplier's available module range and sizes | Designed around your specific garden, requirements and layout |
| Installation speed | Very fast where full access is available | Fast, with assembly time depending on design complexity |
| Thermal performance | Depends on the manufacturer's specification | For U-values / insulation specification, see our specifications page |
| Future alterations | Can be constrained by the original module structure | Can be designed with future changes in mind from the outset |
Our SIP annexes are designed to give you much of the speed and consistency associated with modular construction, without the constraints of a fixed module range:
Not quite. "Garden room" and "granny annexe" both get used for the same type of building, but "granny annexe" specifically implies self-contained living accommodation for a family member, while "garden room" can also cover offices, studios or guest rooms. "Modular" describes how the building is built, not what it's used for - a modular building can be a garden room, a granny annexe, or both at once.
Installation itself can be very fast where full site access is available, sometimes within a few days. However, each installation is unique, so you must confirm this with your own particular build to find out how long in your particular case. There are many on and off site factors that can affect the overall installation time.
No. Crane and lorry access depend on your specific site, including gate widths, overhead obstructions and road access. Always have your access checked before committing to a modular option, since some gardens will only suit a bespoke or panelised build instead.
Often yes, particularly for anything intended for permanent habitation with plumbing and electrics, regardless of whether planning permission itself is required. Check with your council before proceeding, since this varies by project.
"Modular" and "prefab" both describe factory-built sections assembled on site. SIP construction uses precision-manufactured panels assembled into a bespoke structure, rather than fixed pre-built modules - see the comparison above for how this affects flexibility and access.
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