This might be a bit outdated, but we are currently at stage 0, possibly Stage 1.
Stage 7 is when we actually get to use the new extension -
RIBA Plan of Work – The 8 Stages

Stage 0 — Strategic Definition
Establish the project’s need and feasibility.
Consider site options, funding, client requirements.
No design yet — high-level strategy only.
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Stage 1 — Preparation & Briefing
Develop project objectives and constraints.
Prepare the initial Project Brief.
Assemble the project team.
Early cost planning and risk assessment.
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Stage 2 — Concept Design
Initial design ideas and sketch proposals.
Explore options for layout, form, materials.
Set the Concept Design that responds to the brief.
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Stage 3 — Spatial Coordination
(Previously “Developed Design”)
Coordinate architectural, structural, MEP designs.
Designs become more detailed and integrated.
Updated cost plan, planning application usually submitted here.
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Stage 4 — Technical Design
Full technical drawings and specifications.
Finalise all details for construction.
Building regulations and tender documentation prepared.
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Stage 5 — Manufacturing & Construction
Construction works take place on site.
Off-site manufacturing (if any) completed.
Regular site inspections and contract administration.
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Stage 6 — Handover
Building completed and handed over to the client.
Snagging, commissioning, user training.
Preparation for occupation.
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Stage 7 — Use
Post-occupancy evaluation.
Building maintenance and review.
Feedback to improve future projects.