Planning permission controls what a building looks like and how it affects its surroundings. Building regulations control whether it is safe, warm and properly built. They are separate approvals, assessed by different teams against different rules, and most projects need both.
Last checked against current guidance on 2026-08-09. Rules differ by borough — the local pages below say how.
Land use and external impact: scale, appearance, overlooking, daylight, parking, trees and the character of the area.
Planning is decided by your local planning authority against its local plan and national policy. It is a judgement about acceptability, which is why two identical extensions can get different answers in different boroughs.
It says nothing about whether the building will stand up.
Technical performance: structure, fire safety, insulation, ventilation, drainage, stairs, glazing, electrics and access.
Building control is administered either by the local authority's building control team or by a registered building control approver. The assessment is against the Approved Documents:
| Part | Covers | Typical extension issue |
|---|---|---|
| A | Structure | Steel beam sizing over a knocked-through opening |
| B | Fire safety | Escape from a loft room; protected stairway |
| F | Ventilation | Background and extract ventilation in new rooms |
| H | Drainage | Building over or near a public sewer |
| K | Protection from falling | Stair pitch, headroom and guarding |
| L | Conservation of fuel and power | U-values for new walls, roof and glazing |
| M | Access | Level threshold and door widths |
It says nothing about whether the extension is appropriate for the street.
Most structural work needs building regulations approval; only some work needs planning permission.
| Project | Planning | Building regulations |
|---|---|---|
| 3m rear extension within permitted development | Not usually — a lawful development certificate is advisable | Yes |
| 6m rear extension | Prior approval | Yes |
| Rear dormer loft conversion within volume limits | Not usually | Yes |
| Mansard roof | Usually full planning | Yes |
| Internal wall removal | No | Yes |
| Garage conversion (no external change) | Not usually | Yes |
| New window in a flank wall | Sometimes | Yes |
Build cost moves with your builder, your specification and the month you tender. What you can pin down early is the design and compliance scope — how many drawing sets the council and building control will want, and whether structural input is in play. The estimator below works that out from the project type and floor area.
Move the controls for an indicative figure. It anchors your budget — it is not a quote. Every project is priced after a free survey.
Build costs are separate and quoted by your contractor. See our published fees.
Resolve planning first where there is any planning risk, because a planning change redesigns the technical package but not the other way round.
The exception is a scheme that is clearly permitted development and clearly uncontentious, where the two can run together and save several weeks.
Note that a building control application can be made under a building notice or as a full plans application. Full plans gets you a checked design before work starts, which on anything structural is the safer route.
Both problems are cheap to avoid and expensive to fix, which is the whole argument for getting the drawings right at the start.
The body that carried out the building control function — either the local authority or the registered building control approver you appointed.
Wherever loads change: removing a wall, forming an opening, adding a dormer or building a new roof. Building control will expect calculations.
No, it is an additional consent. Works to a listed building can need listed building consent, planning permission and building regulations approval.
Three years from the date of the decision for most permissions, within which the development must be begun.
Planning permission and building control are separate consents under separate statutes, decided by different people against different tests. One asks whether the building should exist in that form and place; the other asks whether it is safe once built. A scheme can pass either and fail the other:
