Older houses usually cost more per square metre to renovate than modern ones, because of solid wall construction, breathability requirements, dated services and, in some cases, protected structure or conservation area status. The extent of specialist work needed is the biggest single variable, so a condition survey should come before any budget.
Traditionally built houses with solid masonry walls behave differently from modern cavity construction. Insulation and repair strategies need to allow the structure to manage moisture, and inappropriate materials can cause damp problems. This is why conservation-aware design advice is a normal cost item on an older property rather than a luxury.
If the house is a protected structure or in an architectural conservation area, works that would otherwise be exempt may require planning permission, and specialist materials and craft trades may be required. The local authority's conservation officer can advise on what is acceptable before you commit to a scope.