Hi Matt!
Extensions have been extremely popular in our area and especially our Indian community people. I challenge anyone to find a Indian owned house that hasn't been extended!!
It's always made sense to 'improve rather than move' in 'most' cases - if your means stays or is likely to stay the same, it makes even more sense.
All those houses built in the 1920's and 1930's which are as you descrbe, 1 Double bedroom, a possible 2nd smaller bedroom and a 'box room' which are cheerfully advertised as '3bedroom houses' are prime candidates for extending. I live in one, we extended it as long ago as 1987 (moved in in 83) and have been extending it bit by bit ever since. We didn't quite go whole hog and slap on a double story rear and side extension, we've only gone single story to the back and side and a complete refurb in 2002. My aunty did go double story and side as they needed an extra large bedroom.
If you look around theres ALOT of extended houses, I'd definitley recommend it if you are planning to stay a little longer and/or how your finances pan out. finding a dream house is possible in the UK if you are prepared to pay some serious amounts for it and of course this being a US and UK site, the comparisons of what you get for your money means it's about 3,4,5 times the price for something similar to a 'dream home' in the US (most parts of the us!)
The renovation work is stressful. No doubt, theres ALOT of arguing, sorting things out, problems, workmen doing work right or wrongly, to contend with - be prepared for long building times from start to final finish - my aunties one took around 2 years start to end (the building work itself took about 2 months! they were damn fast !)
As for adding value - the golden rule is to extend carefully - your property should be worth a max of 20% more than oher homes in your road/area - any more and it wont match the area and possibly leaving your house on the market for longer when it's time to sell ( ie no point having a 5 bed, 3 living room, loft conversion and 3 bathroom house when all nearby homes are 2/3 bedroom semi's)
Some 'additions' will cause your value to actually drop ! - i.e dont extend in a way that you lose a bedroom or dont just add a loft conversion if it's not done by a proper company and doesnt include a bathroom.
Costs - be prepared to be scared! if you 'dont know anyone' then by going with a full building company, it's mega bucks i mean mega pounds! - if you do know some builders or get a recommendation, then you can negotiate whilst getting a good deal. Don't use the super cheap possible rogue traders.
Our 'recently' added side utility room and complete refurb of the house top to bottom cost us around £25K in 2002 of which 5K was the room itself. My aunties large(ish) extension of the same time, double story, extended kitchen, side extension and complete refurb of the whole house was around £75K!
i do know of a house nearby which has fallen into the trap of making the other houses nearby look small and his being worth about 60% more which is an insane double story rear, double story both sides and loft room conversion which is setting him back about £160 K!!! I spoke to the guy after driving past and wondering why so big, so i stopped to chat to him - turns out he wants his 3 kids to have a room each along with his mum & dad so they dont ever have to move again - which is fair enough, but should he ever need to sell, i think he'll have a hard time!
Theres loads more to this, and again it depends on how far you want to go which will be the decider in terms of price and work done - but in most cases is better than moving.
Hope this helps a bit!
Cheers! DtM! West London & Slough UK!