Advice on using RSJ instead of IG Lintel - Advice needed !!!

I am due to start an extension above a double garage. Our structural engineer has advised us to use an
IG Lintel L6/100 Extra Heavy Duty Lintel to span above the double garage door opening (length approx 4.8 metres).
The cost of this Lintel is approx £700 !!
My builder feels that using such a lintel is a little extravagant and a cheaper alternative would be a simple RSJ with a steel plate welded by an ironmongers.
Whilst I do not want to jeopordize the structure of the building by using sub-optimal materials the thought of spending £700 on a lintel shocks me!!
I am waiting to speak to my structural engineer (but he is on holiday at the moment) but does anybody have any experience or advice about the use of RSJ's instead of engineered lintels.
Comments gratefully received !!

Your builder may be right. A steel beam would be more than adequate and much, much cheaper. The only drawback is making it look right.

A structural engineer on a recent job specified catnic box lintels for openings in a 4 storey stairwell - there were lots of them and they cost a fortune. I asked him to check the calulation using 100mm square hollow section. They came out better structurally and were a fraction of the price.

The builder is correct, steelies will be much cheaper, even allowing for fabbing costs for plate and stiffeners, with a shorter lead time too.

And, Jeds, if you'd used UB/UC sections, they would have come out even cheaper: box and channels are far more expensive.

Get the SE to recalc at no extra cost to you. If he baulks at doing it for nothing (it will take all of five minutes), remind him that his professional code (assuming that he is an incorporated or chartered structural engineer) requires him to design with economy and elegance structures to safely resist the forces to which they are subjected .

Thanks for your comments and advice.
Will wait to speak to SE and get him to advise and recalculate with regard to using RSJ.
Only actually spoke to him once and he seemed helpful so hopefully no need to incur any extra costs !!
Thanks again