House Extensions in Hertfordshire and the Home Counties
Designing and building house extensions for private clients across Hertfordshire, Essex and the southern counties. Clear costs from day one, one team from first sketch to handover.
Federation of Master Builders
30+ Years · 100+ Projects
Hertfordshire and Home Counties
Master Builder of the Year
House Extensions, Designed and Built by One Team
At Mosaic Bespoke, we design and build house extensions for clients who want more space, more light, and a home that finally fits how they live, without the cost and upheaval of moving. Stamp duty, fees and the climb up the ladder can swallow the very budget that would transform the home you already own.
As a design and build company we take care of the whole journey: feasibility, architectural design, planning permission, structural engineering, the build itself and the finish. One team, one point of contact, one number you can hold us to.
Costs, timing and planning, answered up front.
What It Costs
As a guide, a high-quality house extension in Hertfordshire typically costs between £2,200 and £3,200 per square metre, depending on complexity, glazing and finish. Use the calculator below for a guide figure shaped to your project.
How Long It Takes
Design, planning and pricing typically take 3 to 5 months before work starts. On site, a single storey extension usually takes 3 to 5 months to build. We give you a clear programme before you commit.
Planning Permission
Many single storey extensions fall under permitted development, so no full planning application is needed. Where consent is required, we prepare the drawings and manage the application for you.
Why Extend Rather Than Move?
A well-designed extension adds the kitchen, bedrooms or living space you are missing, and adds real value at the same time. In this part of the country, quality extension space usually returns significantly more than it costs.
Hertfordshire’s established streets and sensitive planning environment reward extensions that are thoughtfully designed for their setting. Our planning-led approach gives every project the best possible chance of consent, first time.
And because we design and build, the number we agree at the start is the number we hold ourselves to at the end.
Our Process.
Every project follows the same four-stage process, structured to provide clarity, manage planning risk, and ensure your extension is comprehensively prepared before construction begins.
01
Discover
Opinion of Probable Cost
Understand what your extension is likely to cost before committing further.
02
Define
Feasibility Study
Assess planning risk, structure and budget exposure specific to your home.
03
Prepare
Pre-Construction Agreement
Coordinate design, engineering and cost control before your extension begins on site.
04
Deliver
Construction and Delivery
Build with one accountable, structured team, managed daily from start to handover.
See what your extension could cost
What type of extension are you planning?
Choose the option closest to your project.
Answer a few quick questions for an instant guide figure. The estimate is based on typical local build costs; for a detailed figure we will arrange a free consultation.
Every kind of extension, under one roof.
Single Storey and Rear
Kitchens, dining and garden rooms that open the back of the house to light and the garden. The most popular way to transform how a family home lives.
Double Storey Extensions
Two floors from one roof and one set of foundations. The best value per square metre of any extension type.
Wrap-Around Extensions
The side return and the rear combined into one broad, open space. Made for Victorian and Edwardian homes.
Loft Conversions
A new bedroom and bathroom in the roof, often without planning permission, and usually the fastest space to add.
Basement Conversions
A full extra floor beneath your home. Cinema rooms, gyms, guest suites, without losing an inch of garden.
Barn Conversions
Original character, modern comfort. Full conversions of agricultural buildings into remarkable homes.
Frequently Asked Questions
How much does a house extension cost?
As a guide, most quality extensions in this area come out between £2,200 and £3,200 per square metre, plus fittings like kitchens and bathrooms. The biggest factors are size, structural complexity and glazing. The calculator above gives you a guide figure in under a minute.
Do I need planning permission for an extension?
Not always. Many single storey rear extensions fall under permitted development rights, subject to limits on size and position. Larger, two storey or side extensions usually need consent. We check what applies to your home and handle the application if one is needed.
How long does an extension take?
Allow 3 to 5 months for design, planning and pricing, then typically 3 to 5 months on site for a single storey extension. Larger or structurally complex projects take longer. You get a written programme before we start.
Can we live in the house during the build?
Usually, yes. Most extensions can be sealed off from the main house for the majority of the build, and we plan the messy stages around you. We will be honest with you if a project would be easier to live away from for a short period.
Will an extension add value to my home?
A well-designed extension usually adds significantly more value than it costs in this part of the country, especially kitchen and bedroom space. We can talk you through what similar projects have achieved locally.
Considering a House Extension?
If you are weighing up an extension against a move, start with the numbers. Our project appraisal is honest, structured, and entirely without obligation.
Extension builders across Hertfordshire and the Home Counties
We build extensions from our base in St Albans and across Harpenden, Radlett, Potters Bar, Hemel Hempstead and Berkhamsted, and throughout Hertfordshire, Essex and the southern counties. Exploring your options? See our house extensions page, or compare with loft conversions, barn conversions and double storey extensions.