Everyone is naturally keen press ahead and move in before Christmas, but rushing decisions early can be harder to undo later on. It is always best to take time in the early stages to develop a layout you are happy with, and sleep on your decisions before moving to the next stage.
Late changes have the potential to affect any project -and add cost - at each stage. This can occur
after the scheme has been "signed off" ready for Planning, resulting in additional redesign costs not included in the orginal fee
during the Planning process, if the Planners request changes to the scheme
after the Planning Approval, incurring additional design costs and a formal retrospective amendment - or even full resubmission of a new Planning application
towards the end of the detailed design stage, especially if a Structural Engineer has already completed design calculations that need to be redone
during the construction phase after the builder has already commenced construction
Bear mind that 'late changes' can cost time and money at any stage and it is always best to assess and agree the impact of a proposed change before you decide to go ahead with it. If this has an external visual impact, you may need to amend or re-submit the project to the Local Planning Authority.
It is often easier to omit something at a late stage rather than adding it in, so try to consider as much as possible from the outset, for example including other jobs around the house that need a builder, that can be priced from the start.