Project Cost Guide
Planning a house extension or conversion is a major investment, and one of the first questions most homeowners ask is “roughly how much will it cost?”
This page is designed to give you clear, realistic guidance before requesting a full quotation, helping you decide whether your project and budget are aligned from the outset.
Every home is different, and final costs depend on layout, structure, access and specification — but the ranges below reflect typical projects delivered by JB Carpentry Plus Ltd across Hertfordshire.
Typical Project Cost Ranges
House Extensions
Single-storey and double-storey extensions. Rear extensions, side returns and structural additions.
Indicative range:
£60,000 – £200,000+
Loft Conversions
Dormer, hip-to-gable and structural loft conversions, including staircases and full integration into the existing home.
Indicative range:
£45,000 – £90,000+
Garage Conversions
Including insulation, structural alterations and integration with the existing property.
Indicative range:
£25,000 – £45,000+
Larger Renovation Projects
Whole-home reconfiguration or renovation works delivered as part of a larger build.
Indicative range:
£40,000 – £120,000+
What affects the final cost
While the figures above provide early guidance, final costs depend on factors such as:
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Size and complexity of the project
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Structural requirements (steelworks, foundations, roof alterations)
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Level of finish and specification
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Access, site conditions and existing layout
For this reason, we provide clear, written quotations following an initial discussion and site visit.
A quick reality check
If your available budget is significantly below the ranges shown above, we may not be the right fit — and it’s better to establish that early before time is invested on either side.
We focus on well-planned, properly managed residential projects, and aim to be transparent from the very first conversation.
Project Estimate Calculator
The Estimator form below will allow you to peronalise a rough estimate for your project. This helps you and us to understand how much your project will cost, and whether or not that cost is viable.
If Unsure: Length X Width. Example: 6m x 4m = 24m2.
Your estimate will appear here...
Next step
If the figures above feel realistic for your project, the best next step is a short conversation to understand your plans in more detail.
👉 Message us on WhatsApp:
👉 Call us to discuss your project:
(Both options connect you directly with us — no forms, no third-party apps.)
Final note
This guide exists to save time — for you and for us — and to ensure every enquiry starts on the right footing.
We believe good building projects begin with clear expectations, open communication and honest discussion.