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Updated: Nov 22, 2022

At Draw Plans, one of the most common queries we get relates to adding a side extension on a semi-detached house. It seems an easy enough project but unlike a rear kitchen extension the side extension usually gives you so many options in terms of layout and design and that it can make it quite difficult for homeowners to choose a final layout.

If you take a closer look at the sketch above you will see that we added a 3 metre wide side extension on a semi detached house and in this instance we used the front half of the extension to create a study or playroom with a door directly off the entrance hall.

That same room can be used as a home office, a bedroom, a guest room, a games room, a playroom, home cinemas, gym or anything else you can think of as it is well situated with easy access from the hallway.

Another consideration is the addition of another bay window to the front of the extension which would make the room larger and more impressive.

What homeowners fail to recognise immediately with this design is that by adding a side extension you have changed your property into a double fronted house which adds far more kerb appeal and value to the property if designed well.

When it comes to the back half of the extension this is where it gets trickier as there are various options. For the sake of argument we have turned the rear of our side extension plan into the new kitchen area and by stripping out various sections of wall we now have a huge open plan kitchen dining and living area which leads through to the patio and garden.

This a now a family home designed for family living and it a certainty that it will become the hub of the house. And when you need to get away from everyone you still have the front lounge and the office or study.

And before I forget, we added a downstairs w/c also which will be appreciated by all the family.

Now, whilst I this consider this design to be a fairly good use of living space there will be plenty of people that will disagree with me. In fact, there are still quite a few homeowners out there that want the kitchen tucked away like the old days. Some would also argue for a utility room, more storage, wider doors, and that’s fine because there is no one design that will suit everyone.

What I would say is that a side extension on a semi detached house gives you options and it is up to the homeowners to decide where there preferences lie.

Only recently I visited a lady who was building a side extension to their house and whilst talking to her she let it be known that she had already bought her kitchen and a huge island because she loved to cook. When I looked at the design plan for the kitchen I could see that the new kitchen would take up most of the side extension and was enough kitchen for a 10 bedroom house. So, my point is, whatever your preferences are, keep everything in proportion to the actual size of the house.

Another design faux pas that I still struggle with is the idea of 3 toilets or 3 bathrooms in a 3-bedroom house. I blame all the glossy mags and too many designers looking for an easy way to use up space but lots of homeowners are looking for more bathroms than they don't really need.

There is no single layout that works for a 3-bed semi with a side extension. The plan that we show is one that would give you an extra bedroom if you were selling. It turns a standard 3-bed house in a 4-bed semi; that we know will add considerable value and that is before we take into account the kerb appeal of the open plans kitchen and family area.


I spent over an hour hunting on Google to show you the best examples of semi detached single storey house extensiona and I have to tell you that good photos are hard to find.

There are plenty of 2 storey side extension photos but quite a few of them look really bad because they were designed with economy in mind and the end result shows.

When it comes to extending a semi detached house there is no one format that works simply because we have so many different styles. Semi-detached houses have been around for a long time and whilst some of them have a period feel others can look quite cheap and dull.

I’m of the view that modern extension designs can work well on the more period type of semi whilst matching up would be a better way to go on the more modern semi. Ideally, throw in some character and above all, don’t make it look like a property that has been extended.


Q: I have a garage next to my house. Is it a good idea to knock it down and add a side extension?

A: In most cases, a house with a side extension is far more valuable than a house with a garage on the side. If the garage is well built you might consider a garage conversion which is usually cheaper to convert than building an extension.

Q: Should I do a side extension wraparound?

A: If you build a side extension the full length of the house then it is unlikely you will want to do a wraparound as you may be over developing. If you have a garage at the side of your property then it may be worth adding an extension behind the garage and do it as a wraparound. Just be careful you don’t over develop.

Q: How much does a semi detached side extension cost?

A: A 3mt wide by 8mts deep side extension on a semi detached house will cost you in the region of £50k / £70k. There are lots of variables that will have an impact on cost.

Q: Do I need planning permission for a side extension on a semi detached house?

A: Most single storey side extensions will fall under permitted development but you should always check with your local planning office before building.

Q: Will a side extension add value?

A: Yes, a side extension will add considerable value to your property.

Q: How long do it take to build a side extension?

A: Eight weeks is a reasonable period to build a single storey side extension if you give the entire project to a builder. If you project manage and bring in the different trades yourself then it will usually take longer.

Q: Do I need a party wall agreement?

A: You will need a party wall agreement for any extension if you cut into the party wall or excavate within 3 metres of another property.

Q: Should I add a front bay window to my semi detached extension?

A: It depends; sometimes a side extension on a semi detached looks better with a regular window that matches the design of the other windows. If you do add a bay it should match the existing bay exactly.


Before I go I should tell you that Draw Plans are Architectural Technicians based in South London. We specialise in adding space and value at low cost.

If you are considering adding a side extension to a semi detached house then get in touch as we will be delighted to hear from you.

We provide Plans and Drawings for any type of House Extension or Loft Conversion within 7/10 days.

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