Does a two-storey extension need planning permission?

Usually. Permitted development for two-storey rear extensions is narrow, and side or conservation area extensions are more restricted, so most need a householder application.

Will I need a structural engineer?

Yes, for the foundations and for any steel supporting openings formed into the existing house, with approval through building control.

Where can I see costs?

The two-storey extension cost guide covers what drives the price; this page covers structure and planning only.

Planning Permission & Regulations

Two-Storey Extensions: Structure and Planning

Name: Mighty Oak Construction
Address: Tunbridge Wells, Kent, TN1, GB
Telephone: 07850 357962
Price range: ££

Adding a second floor to an extension changes more than the floor area. The foundations carry more load, the upper floor has to tie into the existing house, and the council looks harder at how it affects the neighbours. This guide covers those points; cost and the single versus double storey decision each have their own guides.

What the structure has to do

A two-storey extension puts a second floor, a heavier roof and the upper walls onto the same footprint, so foundations are designed for the full load from the outset. Where the new rooms open into the existing house on both floors, openings are formed in the original external wall, which normally means steel support designed by a structural engineer and approved through building control.

The Wadhurst wall-removal case study on this site shows what forming a structural opening involves: temporary support, steel installation and clean finishing once the beam is in.

How planning treats the upper floor

Permitted development for two-storey rear extensions is narrow, and anything on a side elevation or in a conservation area is far more restricted. Most two-storey extensions therefore go through a householder planning application. The main tests are the effect on neighbours' light and outlook, overlooking from upper windows, and whether the design sits comfortably with the house.

Design choices that commonly help are a roof that steps down from the main ridge, upper windows placed to avoid overlooking, and materials that match the existing house. The existing step-by-step planning guide explains how an application runs and how long a decision takes.

Neighbours and the Party Wall Act

Two-storey work near a boundary frequently triggers the Party Wall etc. Act, particularly where foundations are dug close to a neighbour's or a wall on the boundary line is built up. Notices are served before work starts, and the existing party wall guides explain the notice types and what an award covers. Speaking to neighbours before an application goes in often avoids objections later.