Hi C G, once you have decided on a layout, we can provide you with a budget cost estimate. The budget cost estimate provides a detailed breakdown of works and expenses as well as highlighting potential areas where you can save money. If this is something of interest please send the plans and photos to hello@nestestimating.co.uk and we can provide you with a quote

How far out would the extension go? Doesn't look like very much, but you'd also need to consider where you "land" when you go out of the bifold doors - would you want a terrace at that level? It's certainly not impossible but would require new retaining walls to be built in order to provide the firm footings for the extension and terrace, plus you'd need to think about steps down to the lawn which would be a substantially bigger drop than before. If there's no side access, the groundworks would be tricky as there's nowhere to get a digger in - but perhaps there is access? I'd get an architect round for an initial consultation to see what's possible. Also not sure what width you have and whether that's enough for a run of units, island and dining table - do you know what the width measurement is? Bear in mind you'll need about 600mm for the run, 1000mm (ish) for a gap, 900mm (ish) for an island though it could be a skinny one at 600mm, then possibly 3500mm for dining table and chairs which you can walk around on both sides - so that's about 5700mm minimum if I've done my sums right (though a bench arrangement on one side can minimise the dining space. I seem to remember that @rinked has some handy measurements for minimum sizes of tables & chairs.) But an island is not the be-all-and-end-all of a lovely kitchen - you may be able to create something you love with runs of units and a dining table in the middle. BTW I don't think the garage is actually there, is it? It's just that it's put there on the estate agent's drawing and has stayed on your drawing by accident?

Maybe have a rethink. Have you ditched the original plans for reasons of budget? Two storey extensions are far more cost effective than single. They both need foundations and a roof. A ground floor bedroom won't be as attractive to most buyers as one upstairs. Given budget restrictions I would build a smaller two storey extension.

Since other houses have already extended, I'd recommend looking at what they've done. If asking people to show you round seems too cheeky, a peek at previous sold prices on the big property websites often shows you the listings too. And it's worth a look at your council's website to see if you can access plans from planning permission applications.