Load bearing walls
With regard to the structural stability of the existing walls when undertaking a conversion of a loft space, consideration will need to be given as to how the new loads will be supported. For example, if new floor joists are provided and they are to be supported by an existing wall, the wall will need to continue all the way down through the house to a foundation or alternatively the wall will need to be provided with an adequate intermediate support.
Some houses have through lounges on the ground floor where the existing load-bearing wall that did take a load down to the foundation has been removed, or it may have originally been built as a through lounge, with a steel/timber beam installed over the opening. This beam should be checked to make sure it is strong enough to carry any new loads from the loft conversion that are added.
Generally the additional load (weight) from construction and use of the new loft room(s) does not mean a significant increase on the load being transferred to the foundations.
However, in some cases, the increase in load could be significant and the adequacy of the existing foundations to carry this extra load will need to be checked. It may be necessary for the capability of the foundations to be increased by underpinning them. A structural engineer or your Building Control Body will be able to advise you.