An extension adds new space to a property by building onto the existing structure, while a conversion repurposes existing space within the property, like a loft or garage. Both can increase living space and property value, but extensions typically offer more space and may require more planning permission than conversions. 

Extensions:

• What they are: Extensions add new rooms or enlarge existing ones by building outwards from the existing house. 
• Types: Common types include single-storey, double-storey, side-return, and wraparound extensions.  
• Planning: May require planning permission and building regulations approval. 

Conversions:

• What they are: Conversions involve transforming existing spaces, like lofts, garages, or basements, into usable living areas. 
• Types: Include loft, garage, and basement conversions.