Adding Space – Designing and Building an Extension

This blog is about the design and building of a two-storey extension to the rear of a semi-detached house. The blog will take us through the journey of how the extension goes through the design process, the Planning Application and then onto the actual build. It will tell the story, including all the good and all the less good parts, the changes, the frustration and hopefully a satisfactory end result.

So I was contacted by my Cousin’s daughter, Gemma, who was about to buy a new house with her husband Mike and had plans to initially add an extension to the side of the property. Now it might not always be sensible to carry out work for relatives but I do, I wouldn’t want a relative or anyone else for that matter to get caught out.


Gemma had already sent me a photo of the property and the sizes of the rooms. I decided to be a little proactive and modelled the existing house with a two storey side extension and garage on the ground floor.

We agreed for me to visit the new property once it had been purchased and after cup of tea I proceeded to do a survey of the property, they hadn’t moved in completely and my cousin was busy helping to strip wallpaper for her daughter while an electrician was checking the condition of the electrics.

Starting on the first floor I measured up all the rooms taking photos as I went. Next, a climb up into the loft space to measure the roof pitch etc and take a note of the roof construction. Measuring the ground floor was next and finally the outside of the house. The front and back garden was surveyed together with the driveway to the side.

We had already discussed the general brief over some previous emails and phone calls, a two storey extension providing for a garage and an extra bedroom on the first floor, with a further ground floor addition to the rear to enlarge the kitchen. The driveway to the side was not particularly wide so to build a two storey extension to incorporate a garage on the ground floor was a bit of an ask. Gemma had already had a conversation with the Planning Department and a minimum of 1m was required to the left between a two storey extension and the boundary between the properties.

A rethink was going to be required.

So survey completed, its back in the office to start on working through some different options and layouts. The brief was for a two-storey side extension which would provide for a garage on the ground floor and bedroom on the first floor with an en-suite. A single storey extension to the rear would allow for a larger kitchen and utility room. The kitchen would be open plan with the existing dining room.


It was apparent that a garage on the ground floor was not going to be feasible, there was just not enough available width to squeeze in the width of a garage door, piers, thickness of a cavity wall together with a Planning Requirement for a two storey extension to be one metre from the boundary. The next option explored, was to replace the garage with a reception room with access from the hallway. A shower room would be in place of the existing kitchen. A larger kitchen/diner would be formed by enlarging the existing flat roof dining room. A set of bi-fold doors would give views into the garden.  The new bedroom with en-suite would be accessed from the landing on the first floor. A new dormer window in the front box room would give some welcomed extra room to the front box room.


I felt the main disadvantage of this layout was the width of the reception room on the ground floor and the width of the new bedroom. Externally I felt it sat well with the existing property and would satisfy the Pre Application advice previously obtained. From a practical view, it would allow access from the front garden to the rear and also give room for the connection of new drains and Inspection Chambers to the existing drain run. The utility room would be a bonus for day to day functioning and would hide the washing machine etc from the view of the kitchen/diner. The 3D image below shows how the extension would blend in with the existing property. The front dormer window would give some additional space in the front box room and visually add some interest to the front elevation.


Materials would largely be constrained by the existing. Plain concrete tiles which are red/brown in colour could be reused for the side cheeks of the front dormer and the front portion of the hipped roof. Roughcast render would be replicated on the new walls and PVCu windows and doors would match the existing windows. Incidentally, there is a requirement to provide for three car off-road parking spaces meaning that the driveway will need to extend over the front garden.

It was decided to explore an alternative layout and switch the 2 storey extension to the rear and have a single storey garage at the side. This alternative would allow for a single storey garage to the side with a part pitched and part flat roof. Although I try not to design with flat roofs sometimes there is not an option. The two storey extension would be formed at the rear with part of the kitchen sitting behind the garage. The two storey extension would consist of an extended kitchen with a utility room and on the first floor, the extension would form one bedroom and also form the bathroom.


The garage is set back from the front building line with the side garage wall kept away from the boundary line to allow for fascia and guttering. An external door at the rear of the garage still allows access through to the rear garden. The existing kitchen is converted into a shower room with a toilet and basin. The kitchen is now enlarged to form an open plan spacious and light kitchen/diner accessed through the lounge. The kitchen units are along one side of the kitchen but will have a large island in the centre. Garden views will be had through the bi fold doors. Incidentally, the bifold doors have been reduced in width so that the beam carrying the first floor can sit on the external wall. A window will allow light into the dining area. The utility room is accessed from the kitchen and can be shut off when needed.


Upstairs the bathroom is formed by extending to the side and knocking through the external wall to make enough room for the bath, basin and toilet. This then allows enough width to form access through into the bedroom.

A new dormer will provide some additional space in the front box room. The existing two bedrooms will remain untouched.


3 D images of the front and rear elevations show the completed design that has been agreed. Although garages are not always used as such, there is the option if needed and it will be a good selling point in the future. Access to the rear garden will also be an advantage. The new dormer will sit well and the neighbour has had the same dormer built. The two storey extension will not affect the neighbours in terms of loss of light or privacy. The requirement for a two storey extension to be at least 1m from the boundary has been met and allows space for Inspection Chambers.

Planning permission has been granted, but with conditions! The main condition was that the planners wanted a landscape design to show the extent of hard and soft landscaping. The case officer had suggested that the amount of permeable driveway to be reduced and some hedging at the front to be retained, which was actually in the client’s favour.

My client had contacted the case officer herself with a rough scheme of the proposed driveway and the case officer came back in agreement. New proposed landscaping scheme shown below.

Proposd landscape PNG 2

Once the landscaping scheme had been agreed and the Planning Condition met, Building Regulation Approval would be sought. Planning Permission and Building Regulation Approval are separate issues, dealt with by different departments normally within the same Local Authority. Many lay people get confused with Planning Permission and Building Regulation Approval

Essentially Planners deal with the size, location, how a building will look, the environmental impact together with how the building will impact on the local area as well as many other factors. Certainly not an easy job.

Building Regulation Approval is how a building is constructed and deals more with the technical aspects of the building. The health and safety aspects of a building together with the thermal efficiency and how the building will perform. The Building Regulations are underpinned by a set of Approved Documents which guide the designer and builder. They are divided into different sections, for example, structure, thermal efficiency, fire, hygiene, drainage etc.

A set of drawings were produced for Building Regulation Approval, consisting of floor plans, elevations and sections to show how all the different elements connect with each other. A structural engineer, that I frequently use was employed to determine the sizes of the beams, floor joists and rafters. This package of information is then forwarded to the Local Authority Building Control for approval. Everything was generally all in order, the Building Control Officer (BCO) asked for some additional information on the garage roof construction which was quickly forwarded.

An area that can cause a delay, particularly when building at the rear of existing houses is obtaining a Build Over Agreement.  Where construction is likely to take place within 3m of a public sewer then the agreement of the Local Water Authority needs to be sought. This process is somewhat, disjointed. My client’s drain was at the head of the drain run so was classed as a private sewer but our new extension was just within 3m of the neighbour’s drain.  After a couple of phone calls, it was advised to complete the online enquiry,  (it wasn’t possible to send a set of drawings direct to determine whether a Build Over Agreement was actually necessary). The online enquiry determined that an Agreement was needed. More drawings were produced with details on how the existing pipe was to be protected together with a fee paid. Later it was determined that the Agreement was not needed and the fee was refunded! At least the correct course of action had been followed so everything was good in the end.

Building work has now commenced so it will be interesting to see how things progress in the next few months!