Significant extensions and major refurbishments, delivered through the same structured four-stage process as our bespoke new homes — giving you clarity, cost control, and one accountable team.
A major extension or renovation is not a minor building project. Done well, it can transform how you live in your home and significantly increase its long-term value. Done poorly, it creates disruption, unexpected cost, and problems that are difficult to undo.
At Mosaic Bespoke, we bring the same planning-led, structured approach to extensions and renovations as we do to our bespoke new homes — ensuring that before a single brick is moved, you have financial clarity, design certainty, and a team you can trust.
01
Discover
Opinion of Probable Cost
Understand what your project is likely to cost before committing further.
02
Define
Feasibility Study
Assess planning requirements, structural constraints and financial exposure.
03
Prepare
Pre-Construction Agreement
Coordinate design, engineering and existing structure before work begins.
04
Deliver
Construction and Delivery
Build and refurbish with one accountable, structured team.
01
Discover
Opinion of Probable Cost
1) Opinion of Probable Cost (OPC)
Your first step toward understanding what your extension or renovation is likely to cost.
Before committing to architect fees, planning applications or structural surveys, you need a realistic benchmark. Our Opinion of Probable Cost gives you an experience-based cost assessment aligned with your property, your brief, and any outline proposals — so you can make informed decisions from the outset.
This stage protects you from progressing with incorrect assumptions about budget, scope, or viability.
What's Included?
—A benchmarked Opinion of Probable Cost aligned with your existing property and proposed works
—Initial review of likely planning route — permitted development, householder application, or other consent
—A clear next-step recommendation and strategic plan for moving the project forward
2) Feasibility Study
Once a cost range is established, the project must be properly tested against your property’s specific constraints.
Our Feasibility Study examines the planning context, structural implications of the existing building, specification alignment and programme. For extensions and renovations, this stage is particularly critical — works to an occupied or existing property carry different risks and complexities than a new build on a clear site.
It transforms an idea into an informed direction.
What's Included?
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Planning and Permitted Development Review — Confirmation of consent route, local authority stance, and any listed building, conservation area or green belt sensitivities.
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Existing Structure Assessment — Consideration of how the proposed works interact with the existing building: foundations, party walls, loadbearing elements and services.
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Budget Alignment Check — Testing whether the emerging brief and specification sit comfortably within the established cost range.
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Risk Identification — Early flagging of structural, planning, technical or neighbour-related risks that could affect viability or programme.
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Feasibility Recommendation — A clear, structured conclusion outlining whether to proceed as-is, refine, or reposition the strategy.
02
Define
Feasibility Study
03
Prepare
Pre-Construction Service Agreement
3) Pre-Construction Service Agreement
Prepare the project fully before breaking into your existing home.
This stage is particularly important for extensions and renovations, where works will directly interface with an occupied building. We coordinate design, finalise engineering details, align specifications and establish procurement — so that when work begins, it begins prepared.
The result is reduced on-site disruption, improved cost certainty, and a build that proceeds with clarity rather than reactive decision-making.
What's Included?
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Design Coordination and Technical Alignment — Resolving the interface between new works and the existing structure, ensuring drawings are build-ready.
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Party Wall and Neighbour Notification — Managing statutory obligations where required.
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Refined Cost Plan — The feasibility estimate evolves into a detailed, structured cost plan, increasing certainty before entering a building contract.
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Occupancy and Phasing Strategy — For occupied properties, a phasing plan is established to minimise disruption and define how and when different areas of the home will be affected.
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Construction Readiness Review — By the end of this stage, the project is technically aligned, commercially validated and ready to move into a formal building contract.
4) Construction and Delivery
One accountable team delivering your extension or renovation with structured management.
With preparation complete, construction proceeds through our integrated approach. Because the earlier stages have been structured and controlled, the build phase operates with clarity, minimised disruption to your home, and reduced variation risk.
Execution follows preparation — not assumption.
What's Included?
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Dedicated Project Leadership — A single accountable team overseeing delivery from groundworks through to completion and handover.
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Managed Occupancy — Works are sequenced to respect the occupied nature of the property wherever possible, with clear communication throughout.
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Cost Control and Transparency — Ongoing cost monitoring, variation management and clear financial reporting throughout the build.
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Quality Management and Technical Oversight — Regular site inspections, coordination with consultants and strict adherence to specification.
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Professional Snagging and Handover — Structured snagging, commissioning and formal handover, ensuring the completed works are finished to the expected standard.
04
Deliver
Construction and Delivery
Extensions vs New Builds
What Makes Extensions Different.
Extensions and renovations involve a set of complexities that are distinct from new build projects. Working with an existing structure introduces variables that require careful management from the outset.
- — Planning routes vary — from permitted development to full householder applications, listed building consent, and conservation area considerations
- — Existing structures carry unknowns — foundations, services, loadbearing walls, and material conditions that must be assessed before costs can be confirmed
- — Party wall obligations and neighbour engagement must be managed correctly and early
- — Occupied property management requires a phased, considerate approach to minimise disruption to your daily life
Planning a Major Extension or Renovation?
If you are considering a significant extension or renovation and want clarity before committing, our project appraisal process helps you understand viability, planning requirements, costs, and risks at an early stage.