Specification

Choosing the finishes and fittings, and placing the orders that first fix and second fix depend on.

3 tasks in this phase

Flat plan infographic of a single-storey rear extension split into cooking, dining and living zones, with a utility room and WC placed beside the existing soil stack and walking routes marked between the garden doors and the house

Laying Out a Rear Extension: Open-Plan Living, Utility and WC Decisions Before First Fix

Lay out a single-storey rear extension before first fix: open-plan zones, the utility and WC by the drains, escape and ventilation rules, and socket positions.

Flat infographic of four heating decisions for a rear extension in build order: a heat loss calculation, choosing radiators or underfloor heating, fixing the floor build-up, and sizing and ordering the emitters

Heating a Rear Extension: Radiators, Underfloor Heating or Extending Your Existing System

Choose how to heat a rear extension: radiators, wet or electric underfloor heating, whether your boiler copes, and what Part L asks when a system is extended.

Flat infographic of an extension build programme with long-lead items below it, each working back from the week it is needed through its lead time and a buffer to the date it has to be ordered

Your Extension Specification Schedule: What to Choose and Order Before First Fix

Build a specification schedule for a rear extension: PC and provisional sums, long-lead items, order-by dates and the Part L glazing allowance before you order.

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