Basement Conversions in Hertfordshire and the Home Counties
Adding a full extra floor beneath your home. Engineered waterproofing, party walls and planning, handled by one accountable team.
Federation of Master Builders
30+ Years · 100+ Projects
Hertfordshire and Home Counties
Master Builder of the Year
A Whole New Floor, Hidden Beneath the One You Live On
When a home cannot go out and cannot go up, it can often go down. A basement conversion adds serious, flexible space: cinema rooms, gyms, guest suites, home offices, wine stores, without losing garden or moving away from schools, friends and everything you have built around you.
Basements are also the most technical extension there is. Ground conditions, structural support and waterproofing have to be engineered properly and guaranteed. At Mosaic Bespoke, this is precisely where a design and build team with real structural experience earns its keep.
Costs, timing and planning, answered up front.
What It Costs
Converting an existing cellar typically costs £1,500 to £2,500 per square metre as a guide. Creating a new basement by excavation is a bigger undertaking, typically £3,000 to £5,000 per square metre depending on ground conditions.
Planning and Party Walls
Converting an existing cellar often needs no planning permission. New excavation, light wells or altering the exterior usually do. Party wall agreements are almost always needed, and we manage the whole process.
How Long It Takes
A cellar conversion typically takes 2 to 4 months on site. A new excavated basement usually runs 5 to 9 months depending on size and ground. We set the programme out clearly before work begins.
Why Build a Basement?
It is space that does not exist anywhere else on the plot: a full floor gained without losing an inch of garden or an ounce of kerb appeal.
In high-value areas where every square metre commands a premium, a well-built basement usually adds considerably more value than it costs.
And because the work happens below the house, the character of the street and the home above stays exactly as it was.
Our Process.
Every project follows the same four-stage process, structured to provide clarity, manage planning risk, and ensure your basement is comprehensively prepared before construction begins.
01
Discover
Opinion of Probable Cost
Understand what your basement 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 basement begins on site.
04
Deliver
Construction and Delivery
Build with one accountable, structured team, managed daily from start to handover.
See what your basement conversion 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.
Frequently Asked Questions
How much does a basement conversion cost?
As a guide, converting an existing cellar runs £1,500 to £2,500 per square metre, while a newly excavated basement is typically £3,000 to £5,000 per square metre. The calculator above gives you a starting figure for your home.
Do I need planning permission for a basement?
Often not for converting an existing cellar into living space. New excavation, external light wells and changes to the outside of the house usually need consent. We confirm what applies and handle the application.
What about damp and waterproofing?
Modern basements are waterproofed as an engineered system, designed for the specific ground conditions and backed by guarantees. It is the part of the project we are most rigorous about.
Will I need a party wall agreement?
Almost certainly, if you have neighbours on either side. Basement works fall squarely under the Party Wall Act, and we manage the notices and agreements as part of the project.
Is a basement conversion worth it?
In areas where space commands a premium, a well-built basement usually adds considerably more value than it costs, as well as space you could not otherwise create. We can talk through local examples with you.
Considering a Basement Conversion?
If you are wondering whether your home could go down a level, our appraisal covers ground, structure, party walls and budget before you commit to anything.
Basement conversions across Hertfordshire and the Home Counties
We build basement conversions 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 barn conversions, loft conversions and wrap-around extensions.