People definitely see value in construction at the moment and are prepared to extend their existing homes. I'm currently working on single storey contemporary extension to a 1970's bungalow; the sketch design has been prepared and currently with a local building contractor, who's accessing if the works can be undertaken within the client's budget.
Extensions and renovations, by their nature are more difficult to gauge budget prices for, as existing factors greatly influence the price. Apart from the fact that each extension is unique the following are some of the factor that can influence the project's budget.
Unlike most new builds, access to works involving extensions can be restricted, from no access at all in the case of terraced dwellings, to limited access due to proximity of site boundaries. Depending on the restrictions, these can effect the overall price. The of concrete for foundations, is far greater if it has to be moved from the roadside to the back of a house via wheelbarrows, as opposed to beginning placed directly into the excavated foundation from the back of the readymix truck.
Extensions, by definition will become a part of the existing structure/dwelling and effect the structure in some shape of form. How the extension integrates into the existing structure will have an effect on the project cost. Should the only works in integrating the new and the old be removing a patio door the cost would be significantly less than an extension which is integrated into an existing room (s), as the latter will involve significant demolition and structural works. Works where alterations to existing roofs can also significantly add to the cost.
In general at least some alteration to existing services is required, when extending an existing property. The effects on the project budget of these alterations vary greatly. moving a light switch or socket would have a minimal effect, compared to diverting an underground drain and manhole or relocating the boiler.
Very often when extensions are been built, client's take the opportunity to have other works around the house done while the builder's are in, it may be hanging a few new doors, or putting in a new ensuite or altering partitions, all adding to the overall cost and distort the actual cost of the extension.
Naturally specification can have the biggest influence on the project's cost.