Budget for the connection, not just the room
Start with the rooms you want and what they need to achieve. Then describe where the extension might meet the existing house, which rooms would be affected and what services the new space needs. Treat the sketch as a starting point for assessment, with design, site conditions and approvals still to be confirmed.
Extension budget worksheet
| Part of the project | What to include in the discussion | What remains to be confirmed |
|---|---|---|
| Design and approvals | Drawings, professional input and council requirements. | Who arranges each item and whether it is inside the building price. |
| Ground and structure | Site levels, access, foundations and the proposed form. | What investigations or design decisions pricing depends on. |
| Old-to-new junction | Roof, walls, floor levels and weather protection. | How adjoining rooms and finishes are affected. |
| Services and fit-out | Plumbing, electrical, heating and room finishes. | Exact selections, allowances and specialist trade scope. |
| External completion | Paths, steps, drainage and disturbed outdoor areas. | What the completed site includes beyond the new room. |
Before asking for a detailed price
- Write the essential outcome and a separate wish list.
- Gather property plans and any records of earlier additions.
- Note sloping ground, narrow access and delivery restrictions.
- Identify which existing rooms must remain usable.
- Ask how unconfirmed conditions and changes will be priced.
A bedroom and a bathroom are different briefs
Two extensions with similar floor area can contain very different work. A bedroom brief may focus on storage, daylight and access. An extension containing a bathroom adds wet-area selections and service connections. Record the room functions before using another project’s total as a comparison.
Can I get a firm price from a sketch?
A sketch can support an early discussion, but the level of price certainty depends on how much is known. Ask whether you are receiving an estimate or a quote, what information it relies on and which decisions are needed to make the scope more definite.
Further reading
Guidance checked 30 September 2026. Confirm the requirements and product information for your own project.