A GUIDE TO BGR ANNEXES
BEXLEY GARDEN ANNEXES
ith an annexe you are creating a
warm, fully independent living space
right in your garden which provides
comfortable accommodation for your
grown-up children, older parents or even a private area
for guests. Every annexe made by Bexley Garden Rooms
is completely bespoke, fully insulated and tailored to your
own specific needs.
Our bespoke designs can incorporate everything that
you would expect from a traditional home. Designed as
a self-contained living space, they can include bedrooms,
kitchens, bathrooms and will be fully compliant with
building regulations.
From the first design consultation to the final handover, our
team ensures that every detail is considered - delivering a
space that is practical, attractive and fit for purpose.
CONTENTS
Benefits of an annexe..................................................6-7
Legislation .................................................................. 10-13
Build Process ............................................................... 14-15
SPECIFICATIONS
Foundations....................................................................18-19
Surface water drainage.................................................20-21
Pumping stations...........................................................22
Configuration of walls..................................................23
Configuration of roof....................................................24-25
Configuration of internal ceiling................................26
Electrical installations...................................................26
Windows and doors......................................................28
INTERNAL CONFIGURATIONS
Kitchenette standard.....................................................30
Kitchenette - upgrade...................................................31
Bathroom........................................................................32-33
THE FINISH
External cladding...........................................................34
Flooring...........................................................................34
Heating............................................................................34
The final process............................................................36-37
Contact us.......................................................................38
A
garden annexe is more than just an extra building
in your garden — it is a lifestyle upgrade, a smart
investment and, most importantly, a solution to
many of today’s housing challenges. Whether
you are looking to create an independent space for a loved
one, a secondary dwelling to add value to your property, or
simply enjoy more flexible living, a garden annexe makes
perfect sense.
1. Boost Your Property Value
Adding a high-quality annexe can significantly increase
the market value of your home. Modern buyers are
attracted to properties that offer additional, flexible,
multi-generational living space. As such an annexe
makes your home stand out in a competitive market.
2. A Home for Loved Ones
Allows you to keep your family close without compromising on
independence. They are ideal for elderly parents who want support
nearby but still value their privacy; also perfect for young adults
saving for their first home, giving them space to live independently.
3. Flexibility for Modern Living
They can be transformed into a guest house, gym or creative
studio. Or they can be an extension of your own home, providing
room to work in peace with a dedicated office space. Your
annexe can evolve with your lifestyle needs over time.
The Future-Proof Choice
Investing in a garden annexe isn’t just about the present, it’s
about preparing for the future. It is a smart, long-term decision
that pays for itself in value, convenience and peace of mind.
BENEFITS OF AN ANNEXE
Garden annexes fall under two separate regulatory areas:
Planning Permission and Building Control. Both are managed
by your local authority, but they assess different things.
Any annexe designed for full‑time living — including a
bedroom, bathroom, and kitchen — will require planning
permission. To gain approval, councils generally expect the
space to be used by a dependent relative or immediate
family member, as this keeps the annexe ancillary to the
main home.
Annexes proposed for sub‑letting, Airbnb, or commercial
rental are very unlikely to be approved, as councils view
this as creating a separate dwelling rather than family
accommodation. This also increases the likelihood of the
annexe being given its own council tax band, because
the Valuation Office Agency often treats rental or
holiday‑let units as standalone properties.
PLANNING PERMISSION
10
For these reasons, annexes are best suited for multi‑generational
living, such as elderly parents or adult children saving for their
own home.
Estimated costs for Planning are as follows (based on figures
obtained in January 2026):
•
Should BGR be tasked with the Planning Permission process
on your behalf, this will be charged at circa £1,800.00 (which
would have to be confirmed once BGR have contacted your
local authority).
•
This would not include ad-hoc requirements such as tree
surveys, which might be requested by your local authority.
•
Pricing includes drawings required for Planning and
Building Control purposes, as well as the application
submission.
•
Excludes Application fee, which is charged by local
council and does vary; price is circa £550.00*
(*this figure is only a guide and will need to be confirmed by each local authority).
LEGISLATION
11
Building Control is a legal requirement in the UK and does
apply to residential annexes. Building Control refers to the
system of regulations and inspections that ensure that
construction projects meet specific criteria covering:
The statutory requirements covering:
• Structural integrity
• Fire safety
• Ventilation & drainage
• Insulation and energy efficiency
• Accessibility for disabled people
• Plan Checking
BUILDING CONTROL
12
Before construction begins, plans are submitted to the local
authority or an approved inspector to check for compliance.
Once works commence there will also be regular site
inspections by the Building Control Officers (BCOs or Approved
Inspectors) at various stages throughout the process to sign
off certain elements (e.g. foundations, drainage, completion),
ensuring the work conforms with all regulations.
Once project is complete and approved, a Completion
Certificate is issued confirming compliance, which is crucial
for both selling or insuring the property.
Estimated costings for Building Control are as follows:
•
Building Control approver - £1,450.00
•
Structural Engineer - £1,000.00
•
Professional soakaway - £1,500.00
•
Total circa £3,950.00
13
LEGISLATION
14
here are certain fundamental and key aspects of the
build process, as well as changes to the component
materials, that differentiate a garden room from
an annexe. All in line to ensure compliance with
Building Control and Planning, they are the fundamental
reasons for both the increase in build time as well as the
overall costs of the project.
BGR’s step by step process for the construction of an
Annexe.
14
Initial conversation with the sales team to ascertain
scope of works required
Site visit and full assessment of plot
Project consultation and design to determine final
specifications
15
15
Designs submitted to local authority for planning consent
(decision normally takes a minimum of 8 weeks)
Once planning is granted, project is booked in
Building Control Assessor (BCA) and structural engineer
engaged
Project reviewed and approved by Building Control
Assessor (BCA) and structural engineer
Local water authority contacted for approval of works
(Build Over Agreement)
Commencement of works
BUILD PROCESS
Rather than using ground screws, which is the primary go to
for BGR’s garden rooms, the much more complex trench and
oversite foundations are required for the construction of an
annexe.
Annexe foundations involve digging a trench down to firm
ground, generally to a minimum depth of 1000mm. The
trenches are then filled with concrete, on top of which the
walls are constructed. An insulated concrete slab is then
laid at ground floor level within the building footprint.
A lintel build over is constructed within the foundations
to allow access for service lines into the building.
The foundation works also include the dig out for the
sump pump and approximately 20m of trench work up
to the house for the pipework.
FOUNDATIONS
18
All foundation preparation and the lintel would have to be
approved on site by the Building Control Assessor (BCA).
Cladding
Screed
EPDM membrane
PIR insulation (90mm external)
PIR insulation (60mm external)
200mm x 50mm C24 joists
Vapour barrier
Plasterboard
Plaster skim
Hi-load DPC
Engineering bricks to DPC layer
Concrete trench
18mm OSB
Firring pieces 1:560 fall
DPM
Concrete
Polythene separation layer
PIR insulation (100mm)
Blinding sand
150mm Type 1
Cross battening
Fire boarding
150mmx150mm C24 studwork
PIR insulation (100mm)
Vapour barrier
Insulated plasterboard
Plaster skim
SPECIFICATIONS
19
A professional SOAKAWAY must be included within the build
process for an annexe; this is an engineered underground
drainage system designed to safely dispose of surface water
(rainwater) into the ground in a controlled and compliant
way without causing damage or flooding. This is installed
in line with building regulations to comply with Building
Regulations Part H - Drainage and Waste Disposal,
specifically Requirement H3: Rainwater Drainage.
A
permeability
test
is
actioned to understand the
rate of drainage in the soil
which would ultimately
dictate the size of the
soakaway required.
SURFACE WATER DRAINAGE
20
A surface water soakaway would be buried a minimum distance
of 5 meters from any building and 3 meters from boundaries, built
with modular plastic crates that collect surface water, storing
it in a void space and letting it percolate to the surrounding
ground.
Non-woven geotextile
Polymer soakaway crate
Gravel
Inlet pipe
21
SPECIFICATIONS
A robust, but more effective, mini sewage pumping station
is required for an annexe construction over the more basic
macerator which is commonly used for garden rooms.
Local Waste and Water provider is engaged for approval.
A small submersible pump and rising main is used to
deliver wastewater to the public sewer (or the house soil
vent pipe), eliminating the need for deep, long gravity runs
so that the annexe can be sited where desired.
The walls are constructed with a tanalised timber frame,
the timber size for an annexe increased over a garden
room and switched to C24 (stronger and higher‑quality
than C16, with fewer knots and better load‑bearing
capacity).
22
PUMPING STATION
CONFIGURATION OF WALLS
A high-performance, non-combustible, thermal insulation board
(150mm multipurpose Celotex) is added to the cavities of the
frame rather than the more basic Mineralwool.
Fibre Cement Board, which is an
A1 fire rated sheeting board and
approved by building control for use
in a residential construction, is then
added to the exterior of the frame.
Internally the walls are lined with a
polyethene vapour barrier which stops
moisture from entering the building
structure, protects insulation, improves
airtightness and helps prevent damp
and mould.
The final internal layer is the 40mm
Celotex backed plasterboard which is
finished with a 5mm skim of plaster and a
whitewash.
23
SPECIFICATIONS
Instead of a garden room’s cold
roof design that we would use to
maximise internal head space, an
annexe is constructed with a warm
roof design.
A warm roof is a roof construction
where the insulation layer is
placed above the roof deck and
below the waterproofing layer.
Because the insulation wraps
the structure from the outside,
the deck never gets cold, which
prevents condensation, reduces
thermal bridging and keeps the
building more energy‑efficient.
CONFIGURATION OF ROOF
24
SPECIFICATIONS
EPDM
11mm OSB
PIR
Breathable Membrane
18mm OSB
Joist
Plasterboard
Timber joists are installed and 50mm Celotex insulation added
in between, finished with fire rated expanding foam. The roof
is then lined with a polyethene vapour barrier preventing
moisture transmission through ceiling, again completed
with the standard Siniat Fire Board, 5mm skim of plaster
and a whitewash.
A test certificate will be provided to comply with BS7671
for the Building Inspector for the installation of all
electrical installations at or before the pre-handover
inspection. All in accordance with Part P of approved
documents.
CONFIGURATION OF INTERNAL CEILING
ELECTRICAL INSTALLATIONS
26