Minimum internal wall return on extension

9 Sep 2013
Reaction score
United Kingdom
Hi, I'm not sure if anyone can help on this but worth an ask.

I am building an extension and we have some bifolding doors we wish to install. To keep the doors as wide as possible we want them as close to the outside wall as possible. The builders said normally the rsj needs to sit on a 15cm nib on either end to support it. The structural engineers have said that on the corner of the house there needs to be a minimum of a 550mm internal return (approx 750mm external return). Apparently this is a building regulations requirement. As the structural engineers originally specified the 550mm return, then said 150mm would be ok and are now back to 550mm I would really like to clarify for myself.

Has anyone come across this before and is this indeed a building regs requirement? If so would anyone be kind enough to point me in the right direction on where I can find this written down?

Many thanks
Sponsored Links
Using the Building Regs Part A document the return at the outside external corner of the extension should be 665mm or greater unless the engineer can do some number crunching to prove that a smaller return will work, structurally, which it usually will. Diagram 14 on page 31 (of the PDF) refer to P5 in the diagram.

So without some additional SE calcs Mr BC will expect a 665 return so it begs the question why your SE hasn't stepped up and said he can do some number crunching to prove a smaller return will work, then stick it under the nose of Mr BC and everyone's happy. Though he'll have to spend half hour or so doing some calcs so maybe reluctant if he's watching the pennies. Or maybe he doesn't want to ask you for an extra £20 or whatever.
Freddymercurystwin thank you so much for taking the time to reference that for me, really useful to have a bit of understanding before I go back to them.

The builders have said as you mentioned that normally a smaller return is fine. I'm getting the impression the structural engineers have done whatever they can to do the calcs the quickest they can rather than the best way.

On the document you have pointed to it states the minimum external return should not be less than 665mm. Does it state anywhere that it can be smaller with some calcs from the structural engineer to confirm its safe to do so? The structural engineer did speak with the builder today and mentioned if there was a gust of wind it could take the corner of the house out with a smaller return. If true then obviously we won't want to do it however I have a feeling this is their way of getting out of having to redo the calcs.

Thanks again
As mentioned the 665 return is what Building Control are obliged to accept, however often the engineer can do some calcs to prove that a lessor return will work, these can be shown to Building Control and he is then obliged to accept them and a lessor return. I don't understand your engineers reluctance to do some number calculations to try and prove a lessor return will work, as mentioned unless he is reluctant to expend any more time than necessary or is too embarrassed to ask for an extra few quid to do it (who knows?). It is possible that a lessor return will not work and he has already been through the motions (ie done the calcs) but I doubt it.

Are you doing this on a Notice or did you have full Building Regs drawings prepared and approved prior to starting on site?
Sponsored Links
Ok, thanks. I will have a chat with them tomorrow to find out if they have actually done the calcs to see if a lesser return will work.

Maybe we have been unlucky but this isn't the only problem with them. We also have a 6m wall being removed and when we instructed them they said they would try and get the rsj completely hidden between the ceiling and floor. As it happens they have specified a single beam which will sit under the floor joists and protrude into the room by 25cm (quite a bit). From my understanding none has been hidden between the ceiling and upstairs floor.

Full building regs drawings were done but we went on a building notice. Apparently the building inspector also double checks the calcs but it looks like they haven't done this part yet.
If you instructed the SE to get the beam above the ceiling then that is what they should have done! Albeit a more costly and invasive method that can require disturbance in the upper rooms.
That's my thinking as well. I'm not to fussed about disturbance in the upper rooms as every internal wall (bar 1 as it is structural) is coming out anyway. I will have a chat with the builder and building inspector on site tomorrow and arrange a plan of action before going back to the structural engineers.

Thanks again, really appreciate the help.
The option to conceal a beam within the floor space should be based upon discovery of joist layout and whatever services are likely to clash with the beam. You must also consider what potential services may need pass through this space in the future. It's not easy to get stuff past a steel beam!

If the client has unlimited time and funds then he can have whatever he wishes.

We have just done a very large and awkward beam job. The customer wanted at least one beam concealed within the floor joists and we did it. However the additional costs of re routing the electrics, gas, central heating, phone lines and all the extra work involving joist hangers and web timbers amounted to more than treble the original cost.

Ho hum.
Freddy, old friend,
Serious question. Not knocking you in any way, nor taking the p*ss.
Have you ever had calculations done by a SE to reduce P5 from 660mm return.
If so, how have they done it? Would love to know.
In the past have asked a couple of SE this question, and both answered they could possibly adjust for load, but not for lateral stability and restraint.
Kind regards oldun :?:
Hi Oldun; apologies for butting in here but reducing the size of the return can be done by calculation - it's just that it takes a little time to do.

I recently did one for a two-brick return on the outer face, and BC's checking engineer approved it.

If space is very restricted, it can be handy to reduce the return. It's also easier to prove if it's on the ground floor of a two-storey extension, as the increased load increases the resistance moment of the return.
No need to apologise for your input. We really appreciate it and at least you gave us some idea and did not try to duck the question or bull sh*t your way out of it.
Tony, going to ask you a couple of questions, if you do not wish to answer, then we understand and will pay a SE to give us the answers.
We can understand your point about additional loading above and by possibly doing a reduced pier in engineering bricks for a P5 return, but what other factors should be allowed for. Would it be for lateral restraint, longitudinal restraint or wind resistance.
Out of interest, thinking back, ADA 1992. P5 used to be 385, brick and three quarters. but we always made it two brick. Believe it changed 2004 when cavities got bigger.
Thank you so much oldun
Hi all

Gregory01 what was the outcome. Did you manage to get a reduced buttress approved by building regs? I am in exactly the same position as you and would be grateful for any assistance and advice.

Hi all

Gregory01 what was the outcome. Did you manage to get a reduced buttress approved by building regs? I am in exactly the same position as you and would be grateful for any assistance and advice.

To add to what had been said above, the 665mm rule is from Approved Document Part A, which takes the rule from BS8103-2 - Structural Design of Low Rise Buildings: Code of Practice for Masonry Walls For Housing.

This British Standard is aimed at people without the technical knowledge to carry out structural calculations, and allows them to produce a design acceptable to Building Control. For simplicity many engineers also use this document for guidance - and rightly so.

However, where the client's brief diverges away from the guidance in BS8103, the engineer should be prepared to produce a design based on calculations using a suitable alternative method. The engineer should factor this into their initial price, and if the design changes they have every right to pass on additional costs incurred.

As Tony said above, there are ways of designing walls without returns - one way would be to put a suitable wind post in the cavity which would resist the lateral wind loads and take them into the structure above and below. Your engineer should have no trouble at all designing something to your brief, it's not at all complicated.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links