Do I Need Planning Permission For A Double Storey Extension?

While the double-storey extension options are tighter than a single-storey extension, planning permission is not always necessary.

Garden Extension


In order to build a double-storey house extension, you have to stay within the limitations stated below:

Area: the area of extensions must be less than 50% of the totally free area around the house

Height: for 2 storey extensions, the height must not exceed the height of the existing house eaves. Only single-storey extensions are allowed under permitted development if the distance to the boundary is less than 2m,

Balconies and terraces: they are never allowed within permitted development rights

Appearance: the exterior appearance must blend in with the existing house and the neighbourhood

Length: the max length of rear extensions is 3m for terraced or semi-detached houses, up to 4m for detached houses

Width: the width must be less than 50% of the width of the existing house

Keep in mind that the rules for permitted development are stricter in designated areas.

Architecture Drawings


Even if you don't need planning, after the research is completed, it's nice to get a certificate of lawful development from your local council.

Getting Lawful development from your local council will certify that you've built up within the limitations of approved growth.

The council fee for the lawful development certificate is £103.


When you can not build your extensions under permitted development rights, you'll need to apply for planning permission. Based on the scope of the proposed job, pre-planning may also be part of the process.

Hiring an architect is one choice, and with a freelancer which is even cheaper. Another way to do this is to employ a design and build firm, which is usually the most cost-effective solution.



All building work must include compliance with the building regulations. In the project, qualified building companies partner with the local building control officers or a licensed private agency. They'll give you a building regulatory certificate upon completion.


When you are building or altering a wall that sits astride a boundary with adjacent land, you need a party wall agreement.

When the foundations for your double story extension are excavated, a party wall agreement is required if building the foundations up to 6 m from the boundary, depending on the depth of the foundations.

How Big Can My Double Storey Extension Be?

The limitation regarding how big a double storey extension can be is included in Planning rules.


If the height of a two-storey extension would exceed the existing roof eaves, it is not permitted. However, even if the final ceilings are too low, there are solutions such as excavating a few centimeters on the ground floor.

Leaving exposed joists at the upper floor is a desirable decision to give the room space and air, even when the actual height is lower than preferred.


How far a two-storey extension may project is limited by planning rules.

For example, if your neighbour’s window openings are towards the extension proposed by you, there are limitations to prevent a loss of light to your neighbour’s house. The extension must project no further back than a line set at 45 degrees horizontally from the centre of neighbouring windows.

Reading Sticky Notes

How Long Does A Two Storey Extension Take?

The completion time of a two storey extension can differ substantially. Some external factors such as party wall agreements and planning are especially challenging to predict.

However, it is still possible to assess the average times for each phase. The completion time of the construction, on the other hand, is still relatively easier to predict.

Planning Time

Planning: 8 weeks to 3 months for a successful planning process

Design: 8 weeks

Preparation Time: Technical drawings, building regs, structural design: 4 to 8 weeks

Party wall agreements: from 2 weeks to months, depending on how difficult the negotiation is

Construction Time: On average, the construction of a double storey extension takes 14 to 16 weeks, depending on the size of the project. Even though this sounds a lot of time, however, giving each phase of construction the importance it needs brings up the wanted outcomes.

Excavation and foundations: 1 to 3 weeks

Structural beams and columns: 1 week approximately

External walls: 2 weeks approximately

Ground floor slab and 2nd storey suspended timber floor: 1 week approximately

Roof: 2 weeks approximately

Dry lining and plastering: 2 weeks approximately

Electrics and plumbing: 1 week approximately

Decoration and painting: 1 week approximately

Flooring, skirtings, windows, doors and other joinery: 2 weeks approximately


Types of double storey extensions

There are various types of double storey extensions and they are named according to the relative position to the existing building.

1)Two-Storey Rear Extension

Two-storey rear extension is the double-storey extension type that is positioned at the back of your house. Two-storey rear extensions are usually positioned in the garden.

Two-storey rear extensions are considered as efficient and beneficial solutions for detached and semi-detached houses but also, a terraced house extension at the rear of a property is also sometimes feasible and reasonable.

2)Two-Storey Side Return Extension

Two-Storey Side Return Extension is to use the narrow space at the side of your house, without using up garden space. Two-Storey Side Return Extensions are considered as efficient and beneficial options for ground floor extensions.

Two-Storey Side Return Extensions alter the character and the function of a ground floor without giving up space in the back garden area. Two-Storey Side Return Extensions are considered as efficient and beneficial solutions for detached or semi-detached house extensions.

On the other hand, while building Two-Storey Side Return Extension, one must keep in mind that neighbouring properties with overlooking windows might limit your options in terms of design.

3)Two-Storey Wrap Around Extension

Two-Storey Wrap Around Extensions is creating a combination of the rear house extension and side extension.

Two-Storey Wrap Around Extensions are considered as efficient and beneficial options for semi-detached or detached house extensions, and they may create a very wide addition in the size of your existing property. It is used to create an optimal solution for maximising the added space to the existing property.

4)Two Storey Front Extension

Two Storey Front Extensions are the least prevalent type of the double-storey extensions. All the Local Authorities are applying stricter rules in terms of the alterations that can be made since extending at the front of your property has the biggest and the most risky effect on the appearance.

However, it is not impossible, it can still be feasible mostly for detached houses with a large front garden.

Garden Building

What Direction Should I Extend?

Even though in every project the answer to this question varies, usually the easiest option is extending to the rear. However, that answer is also valid if your garden is big enough for the rear extension.

Side extensions are also considerable options, however, if there are windows from the neighbours’ side, then you might want to reconsider.

It goes on both ways. If your neighbour plans to do a side extension, you might consider the option of filing a planning application together in order to benefit from a bigger space since you collaborate.

For double-storey extensions, hiring an experienced architect or architectural firm would help you understand all the options you have.

Studio20 Architects aims to show you all the options you have in terms of building double-storey extensions, and eventually, to help you to make the best decisions on the direction of your double-storey extension.