Extensions: Costs Per Square Metre and Permissions

What drives the variation

Ground conditions. Foundations near trees, on clay, on sloping sites or over drains cost substantially more. Deep foundations can add a large sum before anything visible exists.

Drainage. Building over or near a public sewer requires agreement with the water authority and sometimes diversion work.

Structural openings. Removing a large section of the rear wall requires steelwork, and long spans with minimal supporting piers require substantial beams and sometimes padstones and columns.

Glazing. Bifold and sliding doors, roof lanterns and large format glazing are among the most expensive components per square metre in a typical extension.

Fit-out level. A shell with plaster and basic finishes differs enormously from a fitted kitchen with stone worktops and integrated appliances.

Permitted development versus planning

Many single-storey rear extensions fall within permitted development, subject to limits on depth, height, materials and proportion of garden covered.

Those rights are removed for flats, in conservation areas for certain elevations, on listed buildings, and where a previous owner's extension used the allowance.

A lawful development certificate confirms that work is permitted development. It costs a fee and provides certainty at sale, which is worth having.

Building regulations approval is required regardless of whether planning permission is.

Party walls

Where the extension is built up to or astride a boundary, or involves excavation near a neighbouring structure, party wall legislation frequently applies.

Notice must be served in the prescribed form and within statutory timescales. Failing to do so can result in an injunction stopping work, which is far more expensive than following the process.

Agreeing with a neighbour informally does not satisfy the requirement in jurisdictions where it is a statutory process.

Timescales

Design and approvals typically take several months before anything starts. Planning determination periods, building control submissions and party wall notice periods all run sequentially rather than in parallel.

Construction of a modest single-storey extension commonly runs three to five months on site, longer with complex glazing or poor weather.

Living through it requires planning if the kitchen is affected, and the temporary arrangements cost money that belongs in the budget.

Contingency and contracts

Hold at least fifteen percent contingency. Groundworks are where the surprises are, and they occur before the project is visibly under way.

Use a written contract with a specification, a payment schedule tied to stages, and a documented process for variations. Verbal changes on site are the main source of dispute.