Many external factors affect the completion time of an extension, such as planning and party wall agreements. However, it is possible to estimate the average time for each preparation phase.
Design, planning and regulations
These processes are the least predictable and can take more or less time than expected. However, the design and planning processes for a two-storey extension will usually take around sixteen weeks.
Obtaining planning permission and party wall agreements can take two weeks to months, depending on how complicated the negotiation is.
Technical or planning drawings, building regs, and structural design usually take four to eight weeks.
Construction
The construction phase is usually much more predictable than the earlier processes.
It takes an average of fourteen to sixteen weeks to construct a double storey extension, depending on the size. Each construction phase has a different duration, and it is crucial to give each phase the appropriate amount of time for the best results.