A double-storey extension adds two floors of new space to your house in one build, typically a larger kitchen or living room downstairs with an extra bedroom, bathroom or en-suite above, sharing one set of foundations and one roof.
We offer a double-storey extension across County Down, and Castlereagh is one of the places we cover, along with nearby Belfast, Newtownabbey and Lisburn. Before anything is agreed we visit, check the existing structure and talk through your options. You then receive a written quote setting out the full scope; for most homes in Northern Ireland it sits within £52,000–£103,000 including VAT.
- Guide price
- £52,000–£103,000 inc VAT
- Time on site
- 14–22 weeks on site
- Postcodes
- BT2, BT4, BT5, BT6 and BT7
- Planning rules
- Northern Ireland
- Local area
- Belfast
- Survey
- Free, with a written quote
What should you budget for a double-storey extension in Castlereagh?
The table below shows guide prices for a double-storey extension in Castlereagh. They are based on rates in Northern Ireland, which run about 14% below the national guide, giving an overall range of £52,000–£103,000. All figures include VAT and are firmed up after a free site survey.
| Type of job | What it usually involves | Guide in Castlereagh |
|---|---|---|
| Small (15–20m² total) | A compact rear or side extension adding a small room on each floor. | £34,500–£56,000 |
| Typical (30–40m² total) | A larger kitchen or living room below with a double bedroom or en-suite above. | £52,000–£103,000 |
| Large (50m² plus total) | Wide or deep extensions, often with a new bathroom, steelwork and high-spec glazing. | £86,000–£155,000 |
The main things that change the figure:
- Combined floor area across both storeys
- How the new roof joins the existing roof
- Bathrooms, en-suites and plumbing on the upper floor
- Scaffolding duration, access and the level of finishes
Scope of the work
- Insulated cavity walls with brick or render to match the house
- First-floor joists, stud walls and new openings upstairs
- Roof structure tied into the existing roof, tiled to match
- Scaffolding for the full build, erected and inspected
- Windows, external doors, plastering and internal doors
- Building control inspections and completion certificate
Local housing and a double-storey extension
County Down takes in suburbs of Belfast, Bangor and Holywood with Victorian and Edwardian villas, and Newry, Downpatrick and Banbridge with older town centres and newer estates. Mourne granite appears in buildings around the mountains, and detached rural homes and bungalows are common across the county, often rendered or pebbledashed.
Villas and semis suit kitchen extensions, loft conversions and garden landscaping. Rural detached homes commonly take extensions and driveways, and rendered houses often need re-rendering, roofline renewal and window replacement.
Whatever era your Castlereagh home dates from, we plan a double-storey extension around how it is actually built rather than a one-size-fits-all specification.
Planning and building rules in Castlereagh
England, Wales, Scotland and Northern Ireland each handle planning a little differently. Castlereagh comes under Northern Ireland, covered below, and we raise any questions with the council before your quote is finalised.
In Northern Ireland, two-storey extensions usually need planning permission from the council.
Building control approval is needed under the Building Regulations (Northern Ireland). There is no Party Wall Act in Northern Ireland.
Rules change and every property is different, so we check the planning history and constraints for your address before we quote.
The four stages
Survey and measure
First, someone visits your home in Castlereagh to see the space, take measurements and note anything that could affect the work, such as access or the condition of what is there. There is no charge for this visit.
Price and scope agreed
We put everything into a written, fixed-price quote: the specification, what's excluded, staged payments and the expected time on site. Take whatever time you need to decide; there's no pressure to commit.
Dates and permissions
Next comes planning the work itself. We set a start date that suits you, line up the right trades, and make sure any approvals for a double-storey extension are in place before the first day on site.
Finish and handover
The job proceeds in the order set out in the programme, with dust and debris kept under control. Finally we tidy away, inspect the work together and pass the finished project over to you with its paperwork.
Working with SK Home Improvements
Questions about a double-storey extension in Castlereagh
Who do I deal with during the job?
One point of contact manages your project from survey to handover. They schedule the trades working under our contract, keep you updated on progress, and are the person to call with any question. You don't need to coordinate builders, electricians or decorators separately, because that is part of what we do.
Can we live at home while it is built?
Most families stay at home during a double-storey extension. The new structure is built outside first, and we only break through into the existing rooms near the end, usually one floor at a time. Upstairs, this may mean a bedroom is out of use for a week or two. We fit dust screens, protect floors and stairs, and plan any time without water or power in advance.
How is the new roof joined to the existing house?
The new roof is usually tied into the existing roof so the two read as one. On a rear extension this often means a lower ridge or a hipped end, with valleys where the slopes meet. We strip back some existing tiles, fix new rafters and battens, and lay matching tiles or slates. Lead or proprietary flashings seal the junctions so water runs off properly.
Will the extension affect my neighbours?
A double-storey extension can affect neighbours' light, outlook and privacy, which is why planning departments look closely at height, depth and window positions. Side windows facing neighbours often need obscure glass. Work near a shared wall may also need party wall notices; the Party Wall etc. Act 1996 applies in England and Wales only. We talk through these points early so the design is realistic.
Can I stay at home during the work?
In most cases, yes. Most people can live at home throughout, and we plan the work to keep disruption down by protecting floors, agreeing working hours and keeping access routes clear. Bigger jobs affecting kitchens, bathrooms or heating may mean a short spell with reduced facilities, and we talk this through with you at the survey.
Nearby towns
Castlereagh and its BT2, BT4, BT5 and BT6 postcodes are part of our coverage. Homeowners in Belfast, Newtownabbey and Lisburn can also book a free survey with us and receive a written price for their job.