• Size i.e. height, width, floor area required
• Architectural layout, required number of rooms, windows, bathroom etc.
• Extent of alterations/changes required to existing
• Suitability of existing structure
• Suitability of roof type
• Extent of mechanical and electrical services, facilities, fittings required
• Specification of finishes, details etc. required
• Location, Travel, distance and time and inconvenience and hence costs.
• Access available – for scaffolding, skips, transport, parking etc.

Though the Building Regulations determine basic parameters of the specification the actual specification may be determined more appropriately by the existing standards and finishes in the house. For example moulded skirting and architrave or a cut string stairs with turned spindles and continuous handrail (without newels) ‘to match materials, style and detail of existing’ would cost significantly more than standard ex stock items and can often involve long lead times.

There is therefore no typical cost or rate per m².
The cost of each proposal has to be estimated based on the above factors and possibly factors or features unique to that specific property. It is common for radically different jobs to be done on exactly similar and neighbouring houses.